Fixing the rear gears in you Gear Cycle

2. 1
   Shift the chain and the shifter both to the lowest possible gear (1st gear), the biggest cog
   
   
3. 2
   Look at the rear cogs (sprockets) and make sure they are not touching the derailleur cog/top pulley (guide pulley) when in the lowest gear. This is very obvious as it causes a very loud noise when the bike is being pedaled in this gear. Note: What the arrow is pointing to is the upper derailleur pulley wheel.
   
   
   • If the cogs are in contact with the cassette, then turn in the derailleur alignment screw ("b" screw or chain gap adjustment screw) clockwise to increase tension until the cogs are separated from the cassette by at least a couple millimeters (1/10").
     
     
   • If the cogs are too far from the cassette, loosen the alignment screw until they touch, then tighten it until they just clear a few millimeters.
     
     
4. 3
   Feel for tension in the lower cable. If there is tension, loosen the tension by turning the cable adjuster (indicated by the red arrow) clockwise until there is none. This is important in preparing for the next step.
   
   
5. 4
   Shift to the highest gear (6th or 7th gear) and observe from behind how the chain rides on the smallest cog.Now observe from behind... If the chain looks like it might come off towards the axle, tighten the "H" screw clockwise until it looks centered or the alignment of the derailleur and the chain on the cog is pretty straight. Likewise, if the chain looks like it's rubbing on the next gear, loosen the "H" screw. To make sure you are adjusting the correct screw, carefully watch the derailleur as you turn the screw, as the derailleur should move with just a fraction of a turn of the correct limiting screw. Then readjust the cable tension until shifting is smooth again.
   
   
6. 5
   Shift to the lowest gear (1st gear) to make sure that the chain does not fall off the cassette. As in the previous step, watch how the chain rides on the cog (this time you're focusing on the largest cog). If the chain seems like it is leaning towards the spokes, tighten the "L" screw clockwise. If it looks like it wants to shift down, loosen the "L" screw. Test your adjustments by shifting through all the gears again.
   
   
7. 6
   Attempt to shift through all of the gears in both directions so you can diagnose whether it is shifting properly. If the bike is having trouble down shifting (shifting to larger rings; chain hugs the smaller cogs) then tighten the cable tension by screwing the tension screw out (counterclockwise) a little bit at a time until it is shifting properly. If the bike is having trouble up shifting (shifting to smaller rings; chain sticks too close to the larger cogs) then loosen the cable tension by turning the tension screw clockwise in a small amount.
   
   
8. 7
   Lubricate the screws and pivot points. Keep the chain lubricated with special chain lube to ensure that stiff chain links do not affect the shifting, and to make sure that the drive train will last.
   
   

   Have fun riding :) and have a safe ride...             Most important : Donot shift the gears randomly or more often as it causes an imbalance in the tension of the gear cable and also by doing this you may even get your derailleur broken down :(                         
   
   Source:
   Wikihow with some modificatations of my own:)