Put the gear selector into any gear other than the absolute lowest or absolute highest gear. Look at the cassette (gear cluster) from directly behind the bike and see if the top pulley on the rear derailleur is DIRECTLY in line with the gear on the cassette. If it is, the chain should run quietly and not skip or jump. If it's not, you'll need to adjust the derailleur cable.
If you have a Shimano rear derailleur, they usually have an adjuster on the derailleur itself right where the housing enters the derailleur. There will also be an adjuster on the shifter itself where the cable housing enters the shifter (at the handlebar).
If the pulley is slightly inside of the gear on the cassette, you will need to screw the adjuster inward (clockwise) until the pulley lines up directly with the cog (gear) on the cassette.
If the pulley is slightly to the outside of the gear on the cassette, you will need to screw the adjuster outward (counterclockwise) until the pulley lines up directly with the cog (gear) on the cassette.
Your drivetrain should run relatively quiet when it's properly adjusted. If that doesn't work or your adjusters run out of 'adjustment room', come back and I'll tell you how to start at square one to easily adjust the rear derailleur.
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