This was posted recently, good website:
http://www.thelibster.com/bass/
Here's a suggestion for coming up with bass lines... Record your songs, maybe even without the bass or just turn the bass down when you play them back. Then try singing or humming a cool bassline over the riffs your band has. Don't worry at all about 'how would I play that'... worry about that later. Just sing or hum or go DA DA BOMP BOMP (just vocalize an imaginary cool bass part) along with your tape. It's a little bit like 'scat' singing, if you know what I mean, except in your own chosen musical style. Then pick up the bass and try to recreate the bass line that you just sang.
Also, think of music as a conversation. Your bass lines should ideally be like a full sentence, with a beginning middle and end... a concise musical thought. Not a run on sentence. Not a bunch of words that are randomly strung together. Put pauses in your groove, the way punctuation works... a space after a concise phrase, that lets you take a breath. And some repetition is good, as if you are talking about something specific, and not just rambling about the beach, pastries, I have a rock in my shoe, my mother used to dress me funny, television shows. Have a theme that recurs, much like a central topic that you are concentrating on. Theme and variation is also a good concept... a main theme that you alter, in an organized way... as if that is your topic.
I like the concept of musical conversation because it encourages you to think about playing musical phrases, rather than merely playing arbitrary individual notes that just so happen to follow one after the next. It also opens you to the idea that you and your band mates are listening to each other, responding to each other, leaving some open space for each other (you don't have to fill up every moment with notes, that's like listening to someone ranting. Give them space to be heard.) For example when your bandmate plays an important riff, you might hang back with more simple part, which lets them come forward more.
I know these are abstract ideas for a newer player, but it's more important that you work on developing interesting musical ideas with your bandmates... because getting your fingers to do the mechanical part is the easy part. That's merely a matter of practicing a lot, every day.
So yeah, learn about chords, but in the meantime try humming your parts as a way to come up with riffs. This is easier when you are not at rehearsal... when you're at home alone and relaxed and can let yourself be creative without feeling self-conscious. You won't be required to know what the notes are named, you just have to think of things that sound cool.