Help:Referencing for beginners
Inline citations: Inline citations are usually small, numbered footnotes like this. They are generally added either directly following the fact that they support, or at the end of the sentence that they support, following any punctuation. When clicked, they take the reader to a citation in a reference section near the bottom of the article.
While editing a page that uses the most common footnote style, you will see inline citations displayed between <ref>...</ref> tags.
If you are creating a new page, or adding references to a page that didn't previously have any, remember to add a References section like the one below near the end of the article:

==References==
{{reflist}}

Note: This is by far the most popular system for inline citations, but sometimes you will find other styles being used in an article. This is acceptable, and you shouldn't change it or mix styles. To add a new reference, just copy and modify an existing one.

RefToolbar: Manually adding references can be a slow and tricky process. Fortunately, there is a tool called "RefToolbar" built into the Wikipedia edit window, which makes it much easier.
To use it, click on  Cite at the top of the edit window, having already positioned your cursor after the sentence or fact you wish to reference. Then select one of the 'Templates' from the dropdown menu that best suits the type of source. These are:

{{cite web}} for references to general websites
{{cite news}} for newspapers and news websites
{{cite book}} for references to books
{{cite journal}} for magazines, academic journals, and papers
A template window then pops up, where you fill in as much information as possible about the source, and give a unique name for it in the "Ref name" field. Click the "Insert" button, which will add the required wikitext in the edit window. If you wish, you can also "Preview" how your reference will look first.
Some fields (such as a web address, also known as a URL) will have a  icon next to them. After filling in this field, you can click it to handily autofill the remaining fields. It doesn't always work properly, though, so be sure to double check it.
Often, you will want to use the same source more than once in an article to support multiple facts. In this case, you can click Named references   in the toolbar, and select a previously added source to re-use.

Using the 2017 wikitext editor: As an alternative to the RefToolbar, it is possible to insert citations in the source editor using a similar automated tool as the one used in the visual editor. For this, you need to enable the 2017 wikitext editor in your preferences. You will then be able to edit the source of pages while inserting citations using the automated tool of the visual editor.

Reliable sources: Wikipedia articles require reliable, published sources that directly support the information presented in the article. Now you know how to add sources to an article, but which sources should you use?
The word "source" in Wikipedia has three meanings: the work itself (for example, a document, article, paper, or book), the creator of the work (for example, the writer), and the publisher of the work (for example, Cambridge University Press). All three can affect reliability.

Reliable sources are those with a reputation for fact-checking and accuracy. They tend to have an editorial process with multiple people scrutinizing work before it is published. Academic and peer-reviewed publications are usually the most reliable sources. Other reliable sources include university textbooks, books published by respected publishing houses, magazines, journals, and news coverage (not opinions) from mainstream newspapers.
Self-published media, where the author and publisher are the same, are usually not acceptable as sources. These can include newsletters, personal websites, press releases, patents, open wikis, personal or group blogs, and tweets. However, if an author is an established expert with a previous record of third-party publications on a topic, their self-published work may be considered reliable for that particular topic.
Whether a source is usable also depends on context. Sources that are reliable for some material are not reliable for other material. For instance, otherwise unreliable self-published sources are usually acceptable to support uncontroversial information about the source's author. You should always try to use the best possible source, particularly when writing about living people.
These are general guidelines, but the topic of reliable sources is a complicated one, and is impossible to fully cover here. You can find more information at Wikipedia:Verifiability and at Wikipedia:Reliable sources. There is also a list of commonly used sources with information on their reliability.
Try it! Take a quiz on reliable sources

See also: 
Wikipedia:VisualEditor/User guide § Adding a new reference
Wikipedia:VisualEditor/User guide § Editing references
Help:Referencing for beginners without using templates
Help:Referencing for beginners with citation templates
Help:Citations quick reference
Help:References and page numbers
Wikipedia:References dos and don'ts
Wikipedia:Identifying reliable sources
Wikipedia:Citation templates
User:Nick Moyes/Easier Referencing for Beginners