Here's a good article on how to adjust the BB5's:
http://www.ecovelo.info/2011/04/15/a-foolproof-method-for-adjusting-avid-bb5-disc-brakes/
I found that my BB5's ended up at their best by following the directions in that article, then adjusting the cable tension - pre-loading the cable by moving the caliper arm a few mm before tightening the nut to hold the cable in place, and then use the barrel adjuster to increase the tension to just before the pads start to rub on the rotor.
If your disc rotor is not true, then use an adjustable spanner to (lightly) bend the rotor back to true - if it's new then likely to be pretty okay without any truing but best to check. The less true your rotor, the farther your pads will have to be from the rotor to avoid rubbing and the worse your brakes will be. After truing you may need to run through the caliper/pad and cable tension adjustments again.
Also, clean your rotor - you can get disc brake cleaner from
Halfords for £5, just spray and wipe with a clean cloth.