Wikipedia:Citing sources
Repeated citations: For multiple use of the same inline citation or footnote, you can use the named references feature, choosing a name to identify the inline citation, and typing <ref name="name">text of the citation</ref>. Thereafter, the same named reference may be reused any number of times either before or after the defining use by typing the previous reference name, like this: <ref name="name" />. The use of the slash before the > means that the tag is self-closing, and the </ref> used to close other references must not be used in addition.
The text of the name can be almost anything‍—‌apart from being completely numeric. If spaces are used in the text of the name, the text must be placed within double quotes. Placing all named references within double quotes may be helpful to future editors who do not know that rule. To help with page maintenance, it is recommended that the text of the name have a connection to the inline citation or footnote, for example "author year page": <ref name="Smith 2005 p94">text of the citation</ref>.
Use straight quotation marks " to enclose the reference name. Do not use curly quotation marks “”. Curly marks are treated as another character, not as delimiters. The page will display an error if one style of quotation marks is used when first naming the reference, and the other style is used in a repeated reference, or if a mix of styles is used in the repeated references.

Citing multiple pages of the same source: When an article cites many different pages from the same source, to avoid the redundancy of many big, nearly identical full citations, most Wikipedia editors use one of these options:

Named references in conjunction with a combined list of page numbers using the |pages= parameter of the {{cite xxx}} templates (most commonly used, but can become confusing for large number of pages)
Named references in conjunction with the {{rp}} or {{r}} templates to specify the page
Short citations
The use of ibid., id., or similar abbreviations is discouraged, as they may become broken as new references are added (op. cit. is less problematic in that it should refer explicitly to a citation contained in the article; however, not all readers are familiar with the meaning of the terms). If the use of ibid is extensive, tag the article using the {{ibid}} template.

What information to include: Listed below is the information that a typical inline citation or general reference will provide, though other details may be added as necessary. This information is included in order to identify the source, assist readers in finding it, and (in the case of inline citations) indicate the place in the source where the information is to be found. (If an article uses short citations, then the inline citations will refer to this information in abbreviated form, as described in the relevant sections above.)
In general, the citation information should be cited as it appears in the original source. For example, the album notes from Hurts 2B Human should not be cited as being from the album Hurts to be Human, or an X (formerly Twitter) user named "i😍dogs" should not be cited as "i[love]dogs". Retain the original special glyphs and spelling.

Examples: 

Books: Citations for books typically include:

name of author(s)
title of book
volume when appropriate
name of publisher
place of publication
date of publication of the edition
chapter or page numbers cited, if appropriate
edition, if not the first edition
ISBN (optional)
Citations for individually authored chapters in books typically include:

name of author(s)
title of the chapter
name of book's editor
name of book and other details as above
chapter number or page numbers for the chapter (optional)
In some instances, the verso of a book's title page may record, "Reprinted with corrections XXXX" or similar, where "XXXX" is a year. This is a different version of a book in the same way that different editions are different versions. Note this in your citation. See § Dates and reprints for how to further information.

Journal articles: Citations for journal articles typically include:

name of the author(s)
year and sometimes month of publication
title of the article
name of the journal
volume number, issue number, and page numbers (article numbers in some electronic journals)
DOI and/or other identifiers are optional and can often be used in place of a less stable URL (although URLs may also be listed in a journal citation)

Newspaper articles: Citations for newspaper articles typically include:

byline (author's name), if any
title of the article
name of the newspaper in italics
city of publication (if not included in name of newspaper)
date of publication
page number(s) are optional and may be substituted with negative number(s) on microfilm reels

Web pages: Citations for World Wide Web pages typically include:

URL of the specific web page where the referenced content can be found
name of the author(s)
title of the article
title or domain name of the website
publisher, if known
date of publication
page number(s) (if applicable)
the date you retrieved (or accessed) the web page (required if the publication date is unknown)

Sound recordings: Citations for sound recordings typically include:

name of the composer(s), songwriter(s), script writer(s) or the like
name of the performer(s)
title of the song or individual track
title of the album (if applicable)
name of the record label
year of release
medium (for example: LP, audio cassette, CD, MP3 file)
approximate time at which event or point of interest occurs, where appropriate
Do not cite an entire body of work by one performer. Instead, make one citation for each work your text relies on.

Film, television, or video recordings: Citations for films, TV episodes, or video recordings typically include:

name of the director
name of the producer, if relevant
names of major performers
the title of a TV episode
title of the film or TV series
name of the studio
year of release
medium (for example: film, videocassette, DVD)
approximate time at which event or point of interest occurs, where appropriate

Wikidata: Wikidata is largely user-generated, and articles should not directly cite Wikidata as a source (just as it would be inappropriate to cite other Wikipedias' articles as sources). 
But Wikidata's statements can be directly transcluded into articles; this is usually done to provide external links or infobox data. For example, more than two million external links from Wikidata are shown through the {{Authority control}} template. There has been controversy over the use of Wikidata in the English Wikipedia due to vandalism and its own sourcing. While there is no consensus on whether information from Wikidata should be used at all, there is general agreement that any Wikidata statements that are transcluded need to be just as – or more – reliable compared to Wikipedia content. As such, Module:WikidataIB and some related modules and templates filter Wikidata statements not supported by a reference by default; however, other modules and templates, such as Module:Wikidata, do not.
To transclude an item from Wikidata, the QID (Q number) of an item in Wikidata needs to be known. QID can by found by searching for an item by the name or DOI in Wikidata. A book, a journal article, a musical recording, sheet music or any other item can be represented by a structured item in Wikidata.
The {{Cite Q}} template can be used to cite works whose metadata is held in Wikidata, provided the cited work meets Wikipedia's standards. As of December 2020, {{Cite Q}} does not support "last, first" or Vancouver-style author name lists, so it should not be used in articles in which "last, first" or Vancouver-style author names are the dominant citation style.

Other: See also:

{{cite album notes}}
{{cite comic}}
{{cite conference}} for conference reports or papers
{{cite court}} for court cases or legal decisions
{{cite act}} for a law or legal act
{{cite encyclopedia}}
{{cite episode}} for TV or radio series
{{cite mailing list}}
{{cite map}}
{{cite newsgroup}}
{{cite patent}} for patents
{{cite press release}}
{{cite report}}
{{cite thesis}}
{{cite video game}}

Identifying parts of a source: When citing lengthy sources, you should identify which part of a source is being cited.

Books and print articles: Specify the page number or range of page numbers. Page numbers are not required for a reference to the book or article as a whole. When you specify a page number, it is helpful to specify the version (date and edition for books) of the source because the layout, pagination, length, etc. can change between editions.
If there are no page numbers, whether in ebooks or print materials, then you can use other means of identifying the relevant section of a lengthy work, such as the chapter number or the section title.
In some works, such as plays and ancient works, there are standard methods of referring to sections, such as "Act 1, scene 2" for plays and Bekker numbers for Aristotle's works. Use these methods whenever appropriate.

Audio and video sources: Specify the time at which the event or other point of interest occurs. Be as precise as possible about the version of the source that you are citing; for example, movies are often released in different editions or "cuts".