Agree. Obsessively listen to and play along with records and learn your heroes' bass parts, that's what worked for me. Repitition until the bass lines were indelibly imprinted in my mind. That and ignoring my high school studies were key.
I would take the advice above that recommends staring with the blues because it's simpler, structure-wise, so easier to focus on your basic mechanics, timing, etc. Plus a lot of Greateful Dead and Phish material is just downright complex by comparison, for someone getting started...