I'd like to disagree with the latter part. I have one on my big toe and one behind my ear, and both of those hurt like a bitch.
As previously stated, anywhere with very litte tissue between the skin and the bone is going to hurt a lot more. And yeah, I've heard the armpit is pretty harsh. My calf really didn't hurt except for when he got close to my shin, and it was a bit sore when he was working on it after I'd already been under the needle for seven hours.
You have to be 18 to sign yourself off, and in most states, you have to be 18 to get one, period. If you have no idea what you want to get and are seriously asking for ideas from others, that is the number one indicator that you are not ready to get a tattoo. No matter how cool something may seem at the spur of the moment, it's going to be there on you for the rest of your life, and there's no turning back. So seriously consider this.
My advice: DO NOT GET A TATTOO.