The cable tension can be adjusted at the barrell adjuster on the derailleur, or at the cable adjuster on the right hand shifter. To decrease cable tension, turn the barrell adjusters on the derailleur clockwise. Often, if your bike is making noise in gear, you may just need a simple cable tension adjustment done at the handlebar or rear derailleur. Try this first. If not, go back and turn in the other direction and check it out.
Low gear is for going uphill, and high gear is for going downhill.
Tighten or loosen this screw until the derailleur shifts the chain to the smallest cog, but does not go past it. Adjust the limit screw, you may only need to turn it slightly. The cable tension should be slack at this point, and you could in fact have the cable disconnected to make this adjustment. Adjust this at the derailleur, and tighten or loosen the barrell adjuster as needed to get it to shift. If your cable was loose here, please cinch it now to tighten it to the derailleur.