I'm still kind of a Linux begginer myself and I started out with Damn Small Linux, then Knoppix (it also doesn't need very much hard drive space) and SuSe. One good thing about SuSe if you're a beginner is that it has YaSt for automatically installing .rpm packages (many programs distributed under that form).
Whatever you decide to use, good luck!

P.S.: Also, if I'm not mistaking, you can also more easily install software in Knoppix 4.0 than in older versions as it can store data on a persistent Knoppix image and combine directories (it gives the impression that you can write in a read-only directory - actually what you see is a combination between the read-only dir and data stored in your computer's memory or on the hard drive). I find it good for experimenting but an installation is still much better.