That's exactly my issue with starting training. I'm very worried about back injuries and want to find a coach to teach me proper form but I've got no idea how to evaluate him.
It's a bit the same conundrum as a non technical founder trying to find a technical founder :-)
Pick up Starting Strength by Mark Rippetoe that's mentioned elsewhere in this thread. Form goes a lot farther when you understand the why rather than just trying to replicate it. The book does an extremely good job of explaining the biomechanics behind the movements and what makes proper form proper.
Having a trainer to make sure what you're actually doing matches what you think you're doing is definitely a good thing, but going in informed may make you less nervous.
I did read Starting Strength, that's what convinced me to make sure that I find a good trainer and I don't trust myself enough to evaluate based on the book...
If you are open to other options than weightlifting you shoul look at www.gymnasticbodies.com it's a gymanstics streght training (no backflips and that just bodyweight strenght), that takes care of mobility, joins preparation, etc from raw raw beginers (I'm talking inclined pushups kind of raw) to iron cross. It has lots of videos and a nice forum with plenty of helpful people.
It's a bit the same conundrum as a non technical founder trying to find a technical founder :-)