jsMath handles a fairly big subset of LaTeX's math mode. If you don't already know LaTeX, you should start there. There are a number of good tutorials. Wikipedia also attempt to render LaTeX math, so you may already be familiar with it from there. We delimit with dollar signs, while they use a <math> tag.

If you already know LaTeX, perhaps you have a more specific question?

Note that you can also view the source of other posts, whch should provideyou with plenty of examples to get started.

Finally, it's not actually necessary to use jsMath (indeed, it wasn't available until recently). It's perfectly acceptable to just write mathematics as you would in an email.