fiddle around synths and sample packs until you find a nice good sound and just play with it really, that's a good start. A midi keyboard would be a good "hands-on" approach when creating any melodies if you're into that sort of thing.
I never start out aiming for a bassline. Generally, I'll open a standalone z3ta+ and just start making wierd noises. 9/10 something interesting happens as I'm mauling the keyboard and I have to quickly open cubase and lay it down before i forget it. After that just experiment a bit, put a few variations and new phrases and build a beat round it.