Practice in the sandbox associated with your user account.
If you are feeling tentative, use Articles for Creation: anyone, whether registered or not, can create an article and submit it for review by more experienced editors before publication.
Find a WikiProject that suits your interests, whether it be a particular subject, citation cleanup, grammar, adding photos, or more. See also the list of Wikipedia backlog projects for areas where you could be particularly helpful.
If you don't log into Wikipedia every day, make sure Wikipedia will send notifications to your email so that you don't miss anything.
Use the watchlist feature (click on stars in the upper right of a page) to keep on top of changes in the pages you find most interesting.
Start big or small: you can create an entirely new article, or start with copyediting, grammar correction, and citation cleanup. All improvements help.
If you're not sure where to start making your edit, or if your edit is appropriate, see if the article's talk page is active and ask there.
Wikipedia's home page contains a Featured Article as well as a "Did You Know...?" section, a selection of facts from recently improved articles. Check both places for examples of good articles or what goes into an "improved" article. The Featured Article of the Day Archive is also a great way to see examples of high-quality Wikipedia content.