Gears on a bike are very useful to adjust the pressure or force needed to go over rough terrain or up hills. Derailleur gears are used on 10- and 12-speed bikes, while hub-and-sprocket gears are used on most 3-speed bikes and some 4- and 5-speed bikes. Bicycles that have gear mechanisms should never be back pedaled, be on the right side when you lay it down, or have the gears shifted when you are not pedaling. If you are unable to shift into the smallest or largest cogs (high or low) then your derailleur mechanism will need to be adjusted.
Using a wall or ceiling mounted hanger, hang your bicycle or have a friend hold it up so the rear wheel is off the ground. Find the adjustment screws. These are located on the rear derailleur. They are probably marked 'H' and 'L' for High and Low. These screws are what keep the derailleur from traveling too far to the left or right. Using a small screwdriver, turn the screws a little bit at a time until the chain centers perfectly on the right cog when you shift and pedal by hand. Do not shift the gears without turning the pedals. Adjust the front derailleur the same way.
If you have hub and sprocket gears that are slipping, you need to adjust the sleeve nut that is located on the end of the toggle chain. Shift the handlebar lever into first gear and turn the nut while you look at the inspection hole. When you see the rivet at the end of the toggle chain is in the center of the hold, shift to second gear. Give the sleeve nut another half turn and shift back to first gear. The cable should be taut so that the lever just manages to shift. Now turn the locknut finger tight against the sleeve nut.
To keep your bike running smoothly, you need to clean and lubricate it regularly. If you ride a lot then you need to lubricate the brake pistons, chain, derailleurs and inner cables every two weeks with a dry spray lubricant. The dry lubricant will not attract dirt. Overall maintenance should be done at least once a year. At this time, you should apply grease to the bearings on the headset, bottom bracket, the hubs and pedals. Most manufacturers recommend water-resistant grease that you may purchase at a hardware store. In 3-speed hubs, you can also add light machine oil in the oil holes.