I've looked around on this site, and I can't find anything about coming up with your own bass lines, which i want to try. Does anyone have any advice on how to do it or where some lessons for it might be.
Just fiddle around, and when you find something that sounds good, write it down. Thats how it works for me.

Learning scales definitely helps a lot.
Find out in what key the song is, use that scale.
'Have you felt the trembling in your stomach, the bass that leads us there'