A tattoo is a permanent addition to your body. Having a tattoo removed at a later date by laser treatment is very costly and can leave scars that look worse than the original tattoo.
Remember: It's illegal for a person to be given a tattoo if they are under 18.
An alternative would be to look into getting a henna tattoo. These are not permanent, and there is no age limit to when you can get one done. The henna will fade within a few months.
Piercings are less permanent that tattoos but can still leave scars and carry a risk of infection.
If you are going to have a piercing, make sure you use a professional body piercer. All piercings require extreme care and some such as the nasal septum and male genitalia can take up to six months to heal properly.