Do everything everyone was saying above. If that doesn't work, this is what I do when I get a troublesome bike. Loosen the high and low limit screws all of the way. Shift the shifter down to the highest gear (smallest cog) or as far as it will go. Now, shift DOWN one (to the next bigger cog). It doesn't actually have to shift on the gears, just change the shifter to the second position. Adjust the tension (either by repositioning the wire or using the barrel adjuster) so that the bike shifts to the second smallest cog and doesn't sound like it's rubbing. Now shift back and forth between the smallest and second smallest cog to make sure it works. It's very likely that it'll shift off of the outside of the smallest cog at this point, so screw in the high limit screw so this doesn't happen. Now shift through the entire cassette and adjust the low limit screw so it doesn't shift off of the biggest cog into the spokes.