Wikis, by their very nature, are perpetually incomplete and forever in need of good editors. To a newcomer, articles may seem reasonable and complete, but in any wiki, of any size, there are bound to be plenty of articles needing attention.

[edit] Steps

Start reading articles. Any articles. Choose topics that interest you and topics you haven't explored before. If nothing comes to mind, look around for a Random Page selection, click it a few times, and see what pops up.
Watch the recent changes list to see what others are editing. You may wish to avoid those particular articles for the moment, as they might result in edit collisions. However, you may find links off one of those pages, and this will lead you to other good articles to edit.
Look or ask around for categories of articles needing attention. These may be stub notices, requests for certain information, requests for cleanup, and so on.
Start a new article by looking at the requested article list. Search wikiHow first if you are working here at wikiHow, because many requested articles have already been written under a slightly different title. We will not need to have Ice a Cake answered if we already have a good article on how to Frost a Cake, for example.
Browse, but make your own list of articles you'd like to edit, or have ideas for. Once you get started, you'll find it's much faster to find or think of things to do than to do them.
Evaluate each article you read. Is the information correct? Does it make sense to you? Would it make sense to somebody just learning that subject? You might even go so far as following the given instructions. You will quickly see if there are missing steps or confusing instructions. And then you will know what to fix. If you can't fix it yourself, flag the article for attention. For all of these flags except perhaps obvious speedy deletes, please explain why you used the flag. The article's discussion page is a good place to say what's wrong. Visit this page to find out how to flag.

[edit] Tips

Learn how to flag articles for attention on the wikis you frequent. If you find something that you don't have the time or ability to correct right now, flag it so that others can give it the necessary attention.
Take the time when you begin to edit a new wiki to read the help and instructions for that project. Conventions vary between wikis and communities, and a concerted effort up front to learn and respect those conventions can build a lot of trust and goodwill.
Ask an administrator or other veteran user for help finding things. Most will gladly point you toward the to-do lists and help start you out on the right foot.
Create an account and log in to edit. Even if you edit under a made-up nickname, it will help you to establish a reputation in the community and use the tools to their fullest.
No edit is too small. Fix a typo or correct a punctuation error whenever you see one, making use of the "minor edit" check box when appropriate.
Be bold (but courteous) in making edits. Editing is what wikis were made for.
Once you start to know your way around, be nice to the new users. When people receive supportive and guiding messages, they are much more likely to stay. And wikis need dozens and dozens of happy writers in order to remain viable.

[edit] Warnings

If you're editing things from Recent Changes, it's courteous to give other editors a few minutes to make additional changes, to avoid edit conflicts.
Don't panic when others change your work. Wikis are supposed to be collaborative.
Don't make controversial edits or edit controversial articles if you are new to a wiki. Learn conventions and establish a reputation in the community first, and then handle with care.
Do not copy materials from other sources unless you are certain that they are in the public domain or freely licensed. Ask an administrator or experienced user for guidance if you want to import something.