Read about scales and such, this website also helps a ton if you don't have that much of a music theory background:
http://www.pianoworld.com/fun/vpc/piano_chords.htm

Just pick a root note, a scale (Minor for example) and the notes displayed down under "Notes in chord/scale" are the notes you're allowed to use for that specific scale, if you're on Ableton I recommend putting in all of those notes on top of each other and filling them on as many octaves as possible, then moving them back so you don't hear them and folding the piano roll, now you can place notes randomly without going off key.

Hope this helps.

Ableton has a "Hip Hop Sub Bass" preset which I've found is heavy as hell. Great for getting a good pump in your song, if you set the decay right.

Was watching this NGHTMRE seminar last night and he also said he uses that sub on almost every track of his, it sounds really good actually and it's so easy to control.