I would second (or third..) what has been offered so far. Pullups are the cheapest and easiest exercise for back in terms of needing equipment and finding somewhere to do them. You can get a portable pull up bar at Wal Mart to hang in any door jam where you live. I have two...( I bought the wife one as well when we were doing P-90X together).
But I would also add that the bent over row with a dumbbell is a great back exercise too. It targets a slightly different area of the Latissimus Dorsi (i.e. back muscle) and recruits other muscles in support in the movement that pullups may not exactly hit. If you can work up to doing this and pull ups...you will be golden.
But if you have to pick one...start with the wide gripe pull ups as your first back exercise. Its a great one...
If you can just get your....mind together....then come on across to me.....
James Marshall (Jimi)Hendrix