Pretty much what unbroken said. Having played guitar you'll obviously find the bass easy to pick up and play but concentrate on being part of a rhythm section. Paying more attention to, and getting a tight rhythm with the drums (bass drum in particular) is a good start. From there, play what sounds good to you, be it just holding down root notes or finding something more melodic - all depends on the songs you want to play and what suits the bands you play in I guess.