I always start with something more important.
I listen to music that represent the feeling I currently have.
Can be slow, fast, specific style etc...
By listening a lot to it, I get many ideas for say... a baseline, drum track, etc...
Sometimes looking at something or hearing a particular sound can spark that interest.
If you really don't get anything, then here's another way:
Enter some random notes using any sound (can be bass, backing or even lead (part of melody)) and continue from there adding stuff. If you don't like the first thing, remove it and see if what remains is a good foundation to get working.
I usually get enough inspiration from that process to start from scratch again (delete all notes/patterns) and start over, but this time with a solid idea.
Also, its more important to find a sound you like. Just play the sound with your keyboard in the instrument or sample section and often enough it will spark some inspiration. Just when you found some nice baseline, try to track it into your pattern, and you have your foundation. Same applies to a drumline of course.