The chains of my bike are clicking and the gear don't work anymore I have had a bike a couple of years and I noticed that my gears broke pretty often. I bought a new one but now that one is breaking too. They are skipping and clicking and I have brought them to different bike repairers so if anyone knows what the deal is with the gears let me know.
If the gears are making noise when you're pedaling it usually is a symptom of the derailleur being out of adjustment.

Two Most Common problems:


Basic derailleur adjustment. In many cases the cable has stretched or something has caused a change in the derailleur cable tension. This is easily fixed with the derailleur cable adjustment barrel which is either on the rear derailleur, or for mountain bikes it's on the shifter body.  Here's a video which will walk you through how to make tune the shifting. 


Video On Rear Derailleur Adjustment


Derailleur hanger has been bent due to an accident or the bike falling over. The hanger is where the derailleur is attached to the bike. A special tool is needed to check and/or fix the hanger. 


There's other stuff which could be wrong but I'd need a better description. Pictures would also help in guiding as mountain bikes have slightly different parts so the details on how to fix will vary.