Tangentially, I advise not walking into a shop and asking to have your bike "tuned up," because the grom you're talking to may just go "OK" and sign you up for a full tune-up, when all you really need is one wheel trued and a rear shifting adjustment.

Your best bet is to ask them (preferably one of the actual mechanics) if they have a couple minutes to look your bike over for issues, and tell them what issues are on your mind ("it makes a weird clunk when I pedal" or whatever). If there's enough stuff that needs work, a tune-up may be cheaper than doing it piecemeal.

Mechanics I've known are generally on your side; they don't want to throw your money away just going through the motions when it's not necessary.

Oh, and "can i do this myself?" What HillRider said: generally yes, with some learning and tools. Park Tool has helpful how-to info: http://www.parktool.com/blog/repair-help If you want someplace to start, look at derailleur position/alignment & adjustment, and adjustment of cable-operated brakes.