As Bloodaxe says 'get out there with other musicians'.
I was lucky enough to be in a bar this evening that had an impromptu blues jam going on. Basically a three piece outfit with the drunner leading but they were asking for other players, be you a drummer, guitarist or a bass player, so I volunteered.
There were a few other musicians went up before me (including a bass player who slapped for two blues tracks!) and I have to say that the quality of playing from the other muso's who did a stint was really good, and the 'house' guys were effing tight! I have to be honest I was bloody nervous when it came to my turn!
I'd never played with these guys before, let alone met them and had a chat about what they might play, but they were really cool. I was introduced to the guitarist who gave me the key and we went from there.
I'm far from the best bass player in the world, and I made a few clangers during my half hour stint, but th fact is I threw caution to the wind and said 'Hey guys, I play bass. Gime a shot'
In my expreience, and people may have different opinions, the best way to learn is by going and doing.
Get out there and play with other folks. It may seem daunting at first but nothing ventured, nothing gained. And that includes f**king up. We've all done it!