Honestly... Any synth or sound-altering device will do - hardware or software. Generally I would recommend something hardware-based inexpensive(!) simple to program, and FUN to play with. Check this out, for example. So fun and cheap... I'm gonna get one!
For me, half the fun of music is exploration. We must all find our own path(s) and musical language with these machines. It wasn't until I bought my first synth and actually played with it that I began to develop a real vocabulary. I learned most every basic thing I needed to from that relatively basic machine. Also... listen to bands you like that make sounds you like - then learn to mimic those sounds somehow on whatever gear you have. I guarantee you will learn a LOT in the process.
I would also recommend checking out Oliver Chesler's great blog: WireToTheEar - lots of fun and informative stuff there. Enjoy - and START NOW!