Ok well I'll come in from a different angle to what most people will say on here.........
I've been playing bass for 30 years on and off (big time off halfway through that 30 years), but I don't know any theory. At all. I don't know what notes I'm playing at any point at all, apart from E A D G, the standard open string tuning.
I picked up a bass because I heard Tina Weymouth, JJ Burnel and Colin Moulding....and because I loved the sound so much, I wanted to have a go myself. I've never been one for learning someone else's basslines note for note (unless it was for a tribute band and it was expected of me), and I can do that no problem, but it's not "FUN". It's not [i]creative[/i].
A lot of people will advise learning basic theory, etc etc, and they're wise words, honestly, but ask yourself.....[i]WHY[/i] do you want to play music on a bass guitar? To be technically great in a few years time? Or because you love music and want to have [i]fun[/i]?
For me it was the later. I learnt to play, by playing along to my favourite music, with my bass, making up basslines that sounded good to me, and I had fun doing it. So I'm still doing it all these years later because I still enjoy it, and I learn everything by ear, and by listening carefully to what the music needs, NOT what amazing techniques I can throw in there.
Theory is probably a good idea to be honest, long term, but don't forget to have fun too, otherwise there's really no point.
Just my opinion, many will shoot me down, but whatever.