Definition: Tongue piercings, while popular and attractive, are often wrought with difficulties. If there was ever a piercing I would advise against getting, this would be it. However, I know they look cool so please at least learn from my mistakes and hopefully you'll avoid the problems that I had.
Tongue piercings aren't complicated to get - it's painful, but not unbearable. Some piercers will say that piercing from the top down or from the bottom up is better, but honestly none of that matters as long as it's pierced carefully and correctly. The myths you've probably heard about dying from the piercer hitting the wrong nerve in your tongue are false, but it can lead to real problems if it's not done correctly.
The barbell your tongue will initially be pierced with should be extra long because your tongue is going to swell up quite a bit. It's imperative that you change the jewelry out to a shorter barbell once your tongue is healed, about 4 weeks after the initial piercing. Having a longer barbell allows for extra play and makes it all too easy to accidentally chomp down on one of the balls while talking or eating. I broke 2 teeth because of this.
Do not smoke while healing a tongue piercing, and do not use harsh mouth wash of any kind. The tar and toxins in cigarette smoke are so bad for your piercing and Listerine or similar mouth washes can actually burn your tongue flesh. I had a white halo around my piercing entrance for months from Listerine burn, and it's still scarred to this day. Cutting the mouth wash with water dilutes the bacteria killing properties, so it's best to use something mild like Biotene.
Tighten your barbell ends every day to make sure it's secure. They often come apart and people accidentally swallow their jewelry, which is usually not dangerous but it can be pretty scary.
Typical starter jewelry is a straight barbell, 16 gauge or larger.