You just need confidence in yourself! Im sure youre a great player! (Also a normal guitar tuner will work fine with a bass guitar).
Just start playing songs you like and then find a band that plays those songs/similar ones as well and youll be set!
Whats string bass.. Is that a normal one or a big one..? I have no idea/ can remember at all..
Also I was thinking of putting up a Song for everyone to learn soon, anyone got any songs/ genres theyd like to see?