My cycle's gears don't change properly
It sounds as though your cable needs to be adjusted.

On the rear dérailleur there will be a barrel adjuster where the cable enters the mech. You can increase the cable tension by rotating the barrel adjuster anti-clockwise and loosen it by (you guessed it!) rotating it clockwise.

By getting the cable tension right you should be able to get more accurate shifting.

Park Tools have a great guide to this on their website. You want section 6 on that page, which deals with indexing (the process of achieving accurate shifting).