I'd get a belly ring, if you're under 18..I got my first tattoo when I was 14, and now that I'm nearly 19, I regret it. I was a different person then, a tattoo is for life, you're going to have htat when you are 60 years old, and LOL from what "desires" said...I don't think a tramp stamp would be a good idea either. Get a belly button ring to start out with, and if you have a tattoo idea, keep that in mind, and if a couple years down the road you still want it, then get it, there's been lots of times when I wanted a tattoo, but months later, I didn't. =]
Get a naval piercing because at least you can take it out if you change your mind later. I wouldn't get a tattoo unless you really have a very specific reason behind it and you have thought about it for at least a year. Its not really as big of a deal to rush into getting a piercing and if you still want that tattoo, you can always get it later, when you have the money and have thought about it more.
Belly ring. A tattoo is permanent and a belly ring isn't. If you don't want the belly ring in the future you can always take it out but a tattoo stays there forever even if you hate it. And whatever you may like now can change several years from now.