I don't think you will find it too terribly difficult; you just need to read up a bit.
You can start with http://wiki.dreamhost.com/Shell. Make sure you read the links in that page as well as the page itself; they will lead you to "good and relevant stuff".
Then, you should probably check out a few of the beginning Linux tutorials (DreamHost runs Debian Linux) that are all over the web (Google will get you set right up), so that you learn how to "run" files, and other "general" stuff.
When you find something in instructions that you don't understand, Google it and read till you do. For installing SquirrelMail plug-ins, it shouldn't take much research.
You can always come back here for help if you have a specific question, but doing a little "basic training" will go a lot further than just getting technical answers here that you might not understand, and the skills you pick up will be useful in the future.