Learn to play the double bass

I play an electric bass (6 string) in a punk band – www.myspace.com/deathofanationuk – and i recently bought a double bass. i found its a lot harder than i was expecting. i have the right hand technique figured, i am slapping it mostly. I’m still having trouble with the left hand. i can’t work out the length of the notes on the fingerboard. i can play the simpler stuff like blues scales etc. but still having trouble using it with the band.