Wikipedia:Citing sources
If the article you are editing is already using a particular citation style, you should follow it; if you believe it is inappropriate for the needs of the article, seek consensus for a change on the talk page. If you are the first contributor to add citations to an article, you may choose whichever style you think best for the article. However, since 5 September 2020, inline parenthetical referencing is a deprecated citation style on English-language Wikipedia.
If all or most of the citations in an article consist of bare URLs, or otherwise fail to provide needed bibliographic data –  such as the name of the source, the title of the article or web page consulted, the author (if known), the publication date (if known), and the page numbers (where relevant) –  then that would not count as a "consistent citation style" and can be changed freely to insert such data. The data provided should be sufficient to uniquely identify the source, allow readers to find it, and allow readers to initially evaluate a source without retrieving it.

Generally considered helpful: The following are standard practice:

improving existing citations by adding missing information, such as by replacing bare URLs with full bibliographic citations: an improvement because it aids verifiability, and fights link rot;
replacing some or all general references with inline citations: an improvement because it provides more verifiable information to the reader, and helps maintain text–source integrity;
imposing one style on an article with inconsistent citation styles (e.g., some of the citations in footnotes and others as parenthetical references): an improvement because it makes the citations easier to understand and edit;
fixing errors in citation coding, including incorrectly used template parameters, and <ref> markup problems: an improvement because it helps the citations to be parsed correctly;
combining duplicate citations (see § Duplicate citations, above);
converting parenthetical referencing to an acceptable referencing style;
replacing opaque named-reference names with conventional ones, such as "Einstein-1905" instead of ":27"; and
making citations added by other editors match the existing style (if any).  Do not revert someone else's contribution merely because the citation style doesn't match.  If you know how to fix it, then fix it.

To be avoided: When an article is already consistent, avoid:

switching between major citation styles or replacing the preferred style of one academic discipline with another's – except when moving away from deprecated styles, such as parenthetical referencing;
adding citation templates to an article that already uses a consistent system without templates, or removing citation templates from an article that uses them consistently;
changing where the references are defined, e.g., moving reference definitions in the reflist to the prose, or moving reference definitions from the prose into the reflist.

Parenthetical referencing: Since September 2020, inline parenthetical referencing has been deprecated on Wikipedia. This includes short citations in parentheses placed within the article text itself, such as (Smith 2010, p. 1). This does not affect short citations that use <ref> tags, which are not inline parenthetical references; see the section on short citations above for that method. As part of the deprecation process in existing articles, discussion of how best to convert inline parenthetical citations into currently accepted formats should be held if there is objection to a particular method.
This should no longer be used, and should be replaced with footnotes if encountered:

Handling links in citations: As noted above under "What information to include", it is helpful to include hyperlinks to source material, when available. Here we note some issues concerning these links.

Avoid embedded links: Embedded links to external websites should not be used as a form of inline citation, because they are highly susceptible to linkrot. Wikipedia allowed this in its early years—for example by adding a link after a sentence, like this: [https://media.guardian.co.uk/site/story/0,14173,1601858,00.html], which is rendered as: [1]. This is no longer recommended. Raw links are not recommended in lieu of properly written out citations, even if placed between ref tags, like this <ref>[https://media.guardian.co.uk/site/story/0,14173,1601858,00.html]</ref>. Since any citation that accurately identifies the source is better than none, do not revert the good-faith addition of partial citations. They should be considered temporary, and replaced with more complete, properly formatted citations as soon as possible.
Embedded links should never be used to place external links in the content of an article, like this: "Example Inc. announced their latest product ...".

Convenience links: A convenience link is a link to a copy of your source on a web page provided by someone other than the original publisher or author. For example, a copy of a newspaper article no longer available on the newspaper's website may be hosted elsewhere. When offering convenience links, it is important to be reasonably certain that the convenience copy is a true copy of the original, without any changes or inappropriate commentary, and that it does not infringe the original rights-holders' copyrights. Accuracy can be assumed when the hosting website appears reliable.
For academic sources, the convenience link is typically a reprint provided by an open-access repository, such as the author's university's library or institutional repository. Such green open access links are generally preferable to paywalled or otherwise commercial and unfree sources.
Where several sites host a copy of the material, the site selected as the convenience link should be the one whose general content appears most in line with Wikipedia:Neutral point of view and Wikipedia:Verifiability.

Indicating availability: If your source is not available online, it should be available in libraries, archives, or collections. If a citation without an external link is challenged as unavailable, any of the following is sufficient to show the material to be reasonably available (though not necessarily reliable): providing an ISBN or OCLC number; linking to an established Wikipedia article about the source (the work, its author, or its publisher); or directly quoting the material on the talk page, briefly and in context.

Links to sources: For a source available in hardcopy, microform, and/or online, omit, in most cases, which one you read. While it is useful to cite author, title, edition (1st, 2nd, etc.), and similar information, it generally is not important to cite a database such as ProQuest, EBSCOhost, or JSTOR (see the list of academic databases and search engines) or to link to such a database requiring a subscription or a third party's login. The basic bibliographic information you provide should be enough to search for the source in any of these databases that have the source. Don't add a URL that has a part of a password embedded in the URL. However, you may provide the DOI, ISBN, or another uniform identifier, if available. If the publisher offers a link to the source or its abstract that does not require a payment or a third party's login for access, you may provide the URL for that link. If the source only exists online, give the link even if access is restricted (see WP:PAYWALL).

Preventing and repairing dead links: To help prevent dead links, persistent identifiers are available for some sources. Some journal articles have a digital object identifier (DOI); some online newspapers and blogs, and also Wikipedia, have permalinks that are stable. When permanent links aren't available, consider making an archived copy of the cited document when writing the article; on-demand web archiving services such as the Wayback Machine (https://web.archive.org/save) or archive.today (https://archive.today) are fairly easy to use (see pre-emptive archiving).
Do not delete a citation merely because the URL is not working. Dead links should be repaired or replaced if possible. If you encounter a dead URL being used as a reliable source to support article content, follow these steps prior to deleting it:

Confirm status: First, check the link to confirm that it is dead and not temporarily down. Search the website to see whether it has been rearranged. The online service "Is it down right now?" can help to determine if a site is down, and any information known.
Check for a changed URL on the same Web site: Pages are frequently moved to different locations on the same site as they become archive content rather than news. The site's error page may have a "Search" box; alternatively, in both the Google and DuckDuckGo search engines – among others – the keyterm "site:" can be used. For instance: site:nytimes.com "the goose is loose".
Check for web archives: Many web archiving services exist (for a full list, see: Wikipedia:List of web archives on Wikipedia); link to their archive of the URL's content, if available. Examples:
Internet Archive has billions of archived web pages. See Wikipedia:Using the Wayback Machine.
archive.today See Wikipedia:Using archive.today
The UK Government Web Archive (https://www.nationalarchives.gov.uk/webarchive/) preserves 1500 UK central government websites.
If multiple archive dates are available, try to use one that is most likely to be the contents of the page seen by the editor who entered the reference on the |access-date=. If that parameter is not specified, a search of the article's revision history can be performed to determine when the link was added to the article.
For most citation templates, archive locations are entered using the |archive-url=, |archive-date= and |url-status= parameters. The primary link is switched to the archive link when |url-status=dead.