If your chain is skipping over the teeth of the cog in 7th gear, it's because your chain and cog are worn. You have to replace both the chain and the cassette or freewheel. I don't know if your bike has a cassette or a freewheel. They are basically the same. Freewheel is the old design where the ratchet is built into the gear cluster. A cassette comes on a more modern hub where the ratchet stays on the hub and you change the gear cluster separately. Some new bikes are still made with the old design, though. Your local bike shop (LBS) will help you select the proper replacements for your drivetrain and tires.