Congratulations! You've just stumbled upon a wiki(a) to which you really, really want to contribute. It has an active community, the environment is stable, and there's lots of content...
Only problem: You're afraid of joining it.
Never fear—from experience, I'm giving you a couple of tips on what to do to make yourself less afraid. There is no guarantee that they will work, however...but hey, you'll never know if you don't try first. ;)
(Disclaimer: I am in no way trying to intimidate you; I'm just speaking from experience.)
1. ObserveFirst things first: observe the wiki(a). Familiarize yourself with its policies, its general style, and its users. Make sure you understand the policies fully, the style well, and the users well also. Be patient, however, as this can take several months, and by that point you may have decided not to join the wiki(a) after all.
Preferably, do not attempt to edit anything during this time unless it's obviously vandalism (e.g. an anon replacing the content of a page with "QWERTYUIOPLKJHGFDSAZXCVBNM"); and even then such an occurrence may not be considered true vandalism if it's a test/sandbox wiki(a).
2. ConverseIf possible, try to talk to active users from the wiki(a) outside of the actual wiki. Community Central Message Walls are a great way to do this, as Central is meant to be a hub for all Wikians as opposed to just those who know lots about a particular topic.
You can ask the users about their experiences on the wiki(a); how much they enjoy contributing; how nice the others are; etc. However, don't be invasive of their time or privacy; just be politely curious. And if they don't respond, don't try to force out responses—after all, there may be a good reason for their silence.
If you still want to join the wiki(a) even after familiarizing yourself with it and talking with its users, then join it! However, try to take it slow at first and always refer back to the site policies if you're unsure of something (and if it's not covered by the policies, take it to an active admin). Once you've grown accustomed to editing, you can always speed up your contribution pace.
You may make careless or minor mistakes early on even after reading through all the policies. However, don't be discouraged; instead, use the experience(s) as ways of learning what not to do in the future.
That's it! If you feel I have left something out, be sure to tell me in the comments below.