You've probably already downloaded (or compiled) a copy of SeaMonkey. This means that you're ready for volunteering to become part of the SeaMonkey testing community. Helping out won't take much of your time, doesn't require special skills, and will help improve SeaMonkey.
You've already downloaded a build. All you have to do is use it as your everyday browser and mail/news reader.
SeaMonkey QA has a page dedicated to ways to get involved with helping. This doesn't involve knowing how to code, although a little knowledge of HTML is helpful. Being involved with QA is good for people wanting to get more familiar with SeaMonkey.
Is there some bug that really bothers you? As well as reporting it, feel free to fix it.
We need documentation for users, web developers, and developers working on SeaMonkey and other Mozilla projects. If you write your own code, document it. Much of the existing code isn't very well documented. In the process of figuring things out, try and document your discoveries.
Do your favorite web sites work properly in SeaMonkey or other Mozilla-based browsers? If not then you can help Mozilla succeed by helping to convince web sites and web developers to develop cross-browser, standards-based content which supports SeaMonkey.
You can help with the Mozilla Tech Evangelism project by filing Tech Evangelism bugs on sites which do not work in SeaMonkey, by triaging existing Tech Evangelism bugs, and by complaining to sites that they do not support your favorite browser!
SeaMonkey developers tend to hang out on IRC. If you've installed ChatZilla, getting connected is easy. Otherwise, fire up your favorite IRC client and connect securely to irc.libera.chat:6697, #seamonkey (a friendly channel frequented by the SeaMonkey developers); so, it is also the development channel. Please note that for general support questions asking first in the forum or newsgroups might be better. Another way to get involved is reading the newsgroups.