If you're a noob as you describe, these programs will not fix the problem.
Yeah, you might get a GB or 3 back (or you MIGHT hideously screw up the entire system -- both programs have the ability to do that!), but that's not the real problem.
The only thing that's actually going to fix this problem is a combination of a larger hard drive and smarter disk management.
In short, you need to either stick a new, larger hard drive in there (and a new larger backup as well of course), and/or you need to make some decisions about what does and does not need to be on your boot drive all the time.
My advice would be to look at your videos, photos, music and "misc" (ahem) collections to see what can be offloaded. iTunes and iPhoto both allow multiple libraries, meaning you could have smaller "portable" libraries and larger "permanent" collections left on the big hard drive at home. Archive your old iMovie/iDVD projects, and try to whatever extent possible to keep sensitive/personal info off your boot drive (in case your machine is ever stolen, saints forbid).
But yes, used WITH CAUTION and BACKUPS, you could save some space by using programs like Monolingual.
As a rule of thumb, you need to keep at *least* 10GB free on today's computers for temp files, virtual ram and misc "overhead." Any less than that and you're askin' for trouble IMHO.