You have four choices for help: documentation, the mailing list, the IRC channel, and Bugzilla. There is some great documentation for Linux users at the Linux Gamers' FAQ, and also at id's original page (though you should no-longer contact them for support). Windows users and Mac users should just Google their errors to check for preexisting solutions. You can try asking on the mailing list and IRC channel.. To file a bug report with Bugzilla, check out our Bugzilla website.
If you've come up with an improvement or fixed something, we'd love to hear about it! Firstly, try to make sure that the patch breaks less than it fixes. We don't require everyone to be decorated Geniuses, but do attempt to produce a patch that you've tested and at list sort-of understand what you're doing.
The preferred way to get the ball rolling on a patch is to file a Bugzilla bug for your request with the patch attached to it and then send a notice to the mailing list about it. If you're really so lazy that you can't do this, we would prefer you at least mail it to zakk@icculus.org rather than not do anything at all.
Please make it clear if the patch you're submitting for inclusion isn't yours. Point
out where you found it and who authored it. This is so we know who to attribute
blame to when it horribly breaks things credit to.
If you know how to code, but never made a patch before, that's okay. Here's the 10ยข survival guide to generating patches.
If you checked the source code out with SVN, the client makes it rather easy to generate patches. In fact it's a function of the client itself. Once you've made a change in your checkout that is not upstream, here is how you use it:
If you didn't get the source code via SVN, or for some reason that doesn't work for your situations, here's how to generate them using the tried-and-trusted diff tool between two files:
For more information on diff, patch, and svn, read their respective man and info pages. For a more in-depth guide on Subversion take a look at the SVN Book.