How to cite a section of a wikipedia article?
This question is very basic and broad in scope; have you made any effort to research it? If you search for "citation reference style" on one of the major search engines, the first few pages are full of useful tutorials.

There are many different formats used for references (e.g., Harvard, APA, Vancouver). If this is for an assignment, ask your instructor which format he or she wants you to use. If this is for a journal article you hope to publish, consult the journal submission guidelines.

Normally, page numbers are not required for short articles. For something longer, such as a book, page numbers (or perhaps chapter numbers or section numbers would be used.