Hello World!
The only thing that I need to say is:
I use GarageBand to make my songs.
I never download loops for my songs, I do my own adjustments in the Software Instruments, then, I play it with my computer keyboard.
If you are a DJ and don't know how to make Wobble Bass, or, You don't know HOW to start you 1st Dubstep song in GarageBand, this tutorial is for you:

Long but helpful.
If you want a pure tutorial just talking about synth, bass, and drum sequences, watch this tutorial by CH1N1AN: