That's enormously little information to go on. I can think of a number of different causes, all with different symptoms and observations. Your best bet is to have it test-driven and inspected at a tire and alignment shop. On the test-drive they will identify a tire pull, torque-steer, or an alignment problem. From the inspection they will find worn control arm bushings, ball joints, tie rod ends, and things like that. They will also "read" the tire wear patterns for clues.