My gears keep slipping!!

SaadR93
SaadR93 Posts: 3
edited February 2011 in MTB beginners
Hello, I bought a shockwave XT 900 from halfords about 3-4 years ago, and I used it almost everyday on dirt trails or in and around a forest, anyway now I live in Manchester and use it for going to the gym and back and it takes around 15-20 min on the bike and its on the road. Recently its been slipping gears where if I pedal hard the gear slips and I bang my knee on the handle bars so how do I fix this? I looked on my bike and that the teeth on the rear gears are almost worn out, as I only use the smallest gear on the back and I think the middle one on the front these are the most worn out. Do I replace the entire chain system or just those gears and how do I find the sizing, is it worth replacing?
Also the the black rectangle thing slips back and forth
Thanks in advance
Here is a picture of the bike with the important bits circled in red
bikele.jpg

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