Bike problem, every time I'm in 5,6 gear the rear derailleur keeps clicking,?

If you are unsure of the problem then could you help me buy a new rear derailleur, will most work with most bikes?

You need to see that it isn’t bent and that it is centered over the top or bottom cog (depending on the derailleur) when the shifter is relaxed.

It’s a matter of methodically adjusting the mechanism, and the late, great Sheldon Brown shows you how here:
http://www.sheldonbrown.com/derailer-adjustment.html

If you are interested in a new derailleur, three questions: how many cogs on the back wheel, what brand of shifters do you have, and what is the model of the current derailleur?

Had similar problem, with clicking around the sproket. Took my bike to repair shop assuming it was some ball bearings that needed to be fixed and it was only a matter that the sproket needed to be tighted. Try making everything is tight before spending any money.