Let me be the first to welcome you to TB. I, too, just joined and I'm finding the forum to be quite helpful.
As to your question, there are several books I bought, either at a book store or at Amazon.com, that I found to be helpful. Bass Guitar For Dummies and The Complete Idiot's Guide to Playing Bass Guitar. The most important lesson you can learn is to memorize the fretboard and to learn all the different ways of playing riffs.
I hope this helps.