There are lots of folks who can adjust stuff on their bikes themselves. But, I've had much better success with the guys at the corner bike shop. Now that I've built a relationship with them, they will often stop what they are doing to make a quick adjustment to my brakes or shifters. If it is a job that just takes two or three minutes (and many adjustments take a skilled tech less time than that, they often charge me nothing.
I'd suggest having a tech at a bike shop in your neighborhood adjust the shifting and the brakes, check the wheel truing, and just look over the bike for any loose bolts or bearings that need adjustment. If no problems are found, this sort of "light" tune-up should cost less than $50.
The BEST reason to buy a bike from the bike shop on the corner, rather than from Wal-Mart, is that these sorts of adjustments are done before you ride off on the bike. And, most bike shops ask you to bring the bike back in a few weeks for another "free" tune-up. During the break-in period, cables stretch, wheels go out of true, bolts get loose...so good shops want to look over your bike after you have put ten or twenty hours of riding into it.
Why does the "best" bike at Wal-Mart sell for $150 when the cheapest bike at a neighborhood shop sells for closer to $300? A good shop puts as many hours into assembling, adjusting, fitting, and tuning a $300 bike as they would a $3,000 bike. That investment of time by skilled techs is the difference between a bike that is barely rideable, and a bike that is a joy to ride.