Don't let the shop intimidate you when you first walk in. For the un-inked, a tattoo shop is intimidating enough. Strange smells, strange sounds. Some shops even try to look intimidating to create a tough-guy feel. Just keep in mind that you're a potential customer. Consider it window shopping.
The first thing you should do is to take a minute to look around. Chances are, you'll encounter some flash (stock illustrations) stapled on the walls. These will most likely lean toward the traditional. Skull and crossbones, roses and the like.
You might also see some signs ("No minors; we ID," "We have sanitary conditions" etc.). These signs will also be indicators of the personality of the shop owner. If the signs seem overly intimidating, patronizing or snobbish, they can be tip-offs of the shop's attitude. Some are very friendly, with plants, aquarium fish, and signs like "Tattooed people come in all colors."
Note: There is no national law regarding the legal age for tattooing. Check with the shop to find out what the local statute regulates.