Sources are usually things like birth certificates, marriage certificates, death certificates, quotes from newpapers or family history books that back up what you've put in the data section. Some people post actual scans of the documents, if they have them, or you can quote another reliable source, such an online database frorm the state archives.
There's a help file here :
https://www.wikitree.com/wiki/Category:Sources
Also, click on the "Edit" tab on the profile page and a text box will open where you can add biography and sources. There are some examples of sources to the right of that box.
Hope this helps.