The Help:Sources section already mentioned is a good place to start. The Chicago Manual of Style is the preferred format and if you really, really want good genealogical citations, the book Evidence Explained by Elizabeth Shown Mills is probably the gold standard. There is an accompanying website that has examples in it as well.
The purpose of a citation is to guide someone else to the same sources. A simple link to a web page is better than nothing but it could be completely broken next week thus not being helpful. Some sites like FamilySearch and FindAGrave giver reasonable citations that can be copied and inserted.
