i like to mess around with chords melodies and the bassline. When i get one of them doing something cool - say its the melody first i will go and make the bassline relate to it with its sound design. Sometimes you want the melodies to contrast a bit with the pads, sometimes you want to morph them together to make them sound like one entity. its a difficult one. its the most fun part with production (besides talking like a robot through a vocoder) hope this helps.
Normally if I go in with a set idea it never really works out, I will start using the idea then accidently do something that I think sounds decent and then it all starts to build from that. Most of the stuff I come up with is just from messing about. Sometimes it's good sometimes not so good.
Naa man, you aint the only one. its all about stayin focused...
but yeah, when i started, i felt the same way, lookin at that empty screen is daunting, i now just sit down, import a decent break as a starting point, try t make a quick reese patch, bounce that, then sample it, lay a quick "simple" bassline down, and then repeat the process for a melody and sub, once i have all aspects of the tune, ill then spend time trying to fit it into a decent tune, sometimes it works sometimes it doesnt... if it does then i add more to it and just keep working and if it doesnt, scrap it and start again. I find though if you take too long with each part of the mix, it kills the vibe of the session and you get nowhere. just have to keep at it!
if you know anything about musci theory it tends to be a tad easyer, but loadsa harder at the same time. pick a note, most people go with A, C, D, E, or F, or G, or B or anything acutlay, and just bass it all round that. just start every 4 or 8 bars on that same note, and just mess about on your midi keybord/piano roll til your happy if you want something a little dark chuck in sharps and flats, or chromatic scale if you wanna use the proper name. if you wanna use proper scales/keys, just google say "D Major scale" and there shud be a wikki page with the scal on it, and all the notes from it! but generaly. Then make a bassline. simpler version of it for the sub. more complex for the melody. or what ever the fak you want. dont forget, Dnb tends to be a break beat with some sub.. trhen anything gose!!