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.
If you are worried something doesn't look right you could speak to a school nurse for support and advice - they offer weekly confidential 'drop-ins' in secondary schools. Send a text to ChatHealth (07507 333356) or ask in school to find out more.