Rebuild Gary Fisher Mullet rear derailleur I have a Gary Fisher Mullet from sometime between 2006 - 2008. I don't know anything about bike except the fact that I got one Hehe. 

My rear derailleur is in pretty bad shape. Well, I don't know for the derailleur itself but the gears are smashed. 

I barely use the bike and I don't bike that much at all. But it's summer and I wan't to at least do some runs with it because it is almost brand new.

The derailleur is a Shimano, not sure about the model, but it's the one that came with the bike, so that might help. 

I'm trying to keep the cost as low as possible, so I was wondering if I can buy the gears without changing everything, and then go to a bike shop and have someone ajust it ? Or do I need to buy a complete system for it ?

Thank you for your inputs, Greetings.
For explanation of bicycle parts see: Terminology index - a list of bike part names and cycling concepts

The problem you describe may be caused something other than worn sprockets. It may be 'ghost shifting' where the chain hopes back and forth between two cassette sprockets. This may be caused be misalignment or bad adjustment of the derailleur or gear shifters.

I would take your bike to a good local bike shop, have them diagnose the problem and find out what components - if any - are worn out. You may find that you simply need a good tune-up. 

Good news is you can replace the cassette and chain, derailleur etc. individually without having to replace the whole drivetrain (unless the whole drivetrain is worn out of course!)