Here are some useful resources that you might not know about.
Wikipedia Manual of Style This is the main document defining Wikipedia's "house style". Familiarity with it is essential when editing articles to become Featured Article Candidates (FAC) or Featured List Candidates (FLC). It is very large, and most editors will prefer a more gentle introduction.
Wikipedia Simplified Manual of Style This is it. It is a short and very readable introduction covering the most important points, with many links to documents which amplify each point.
Basic copyediting Copyediting involves the "five Cs": making the article clear, correct, concise, comprehensible, and consistent. The following is a guide for new copyeditors.
The Guild of Copyeditors This is a page for those interested in the copy editing side of Wikipedia. A fine and upstanding bunch of Wikipedians who all know the difference between it's and its.
List of shortcuts This is a directory of abbreviated redirects and the pages they lead to.
Wikipedia policies and guidelinesWikipedia policies and guidelines are developed by the community to describe best practice, clarify principles, resolve conflicts, and otherwise further our goal of creating a free, reliable encyclopedia.
Reliable Sources and how to identify them.
Reliable sources noticeboard This is the place to go to seek help if you want to know if a source is reliable, or if you are in dispute with another editor or editors about a reliable sources matter.
How to cite sources This page explains how to cite your reliable source once you have found it.
Resource Request Project The resource request page is where you can go to ask for a source if you can't find one yourself.
No original research How to avoid original research. This also has a usefull section on primary and secondary sources.
Neutral Point of View This page describes Wikipedia's policy on neutral points of view and how to maintain one when editing.
Barnstars If you want to thank someone for their great work at Wikipedia, in whatever capacity, then you can give them a Barnstar. Spread the love. You know you want to. :)
Cleanup Templates If you think an article has issues and needs help then consider adding one of these tempates rather than simply deleting stuff. These templates can draw attention to a wider audience that articles need improving. For instance one can visit Category:Wikipedia_cleanup and see a selection of articles that need work of some kind or other.
Wikipedia:EL How to use external links on Wikipedia.
Wikipedia Help Last but definitely not least.