I presume you have the original Avid BB5s that came with the bike. Well, here's what I suggest...
Option 1) Upgrade them to the much more easily adjusted BB7s!
Option 2) Google for a technical manual on Avid BB5s. Learn, digest, do!
Option 3) Read below and try it out (seriously though, the above two options are better!)
The outside dial of the brake disc can be turned by hand to bring the brake disc on that side nearer the rotor. This will tighten up the pull on the cable. It's only too close when it starts to rub. The dial is red, and has arrows that say 'in' and 'out'. In brings the discs closer, 'out' moves them away.
To adjust the other side, you then have to loosen the two bolts mounting the braking system to the frame, then pull the brake tight, hold it there and retighten those bolts.
Much, much fiddling, squinting and listening whilst repeating the above until those discs are dialled in so tight that you can't fit a midges wing between the disc and the rotor give you the best you can get.
The BB7s are better as you have a hand turn disc on both sides - much less fiddly
