Is it all the time? If it is then the wheel probably isn't in straight. Loosen off the wheel nuts and pull the wheel into the middle, then tighten them back up. It's also important to make sure the axle is sitting in the frame.
If it is only at one point on the wheel, then either the tyre isn't on properly, or the wheel itself is buckled. If the tyre is wobbling around but the wheel seems to be straight you basically deflate it, wiggle it about a bit at the point the tyre goes sideways to get it sitting on the rim right, and reinflate it.