You're in college, you're 18, you don't need mommy and daddy's permission for everything anymore. Be a rebel, and do it because you can.
The day I turned 18 I got my nose pierced. Two weeks after moving into college I got my first tattoo. College is the fun, exploratory time in your life. I'm not saying go out an ink your whole body. Tattoo's are permanent so be sure it's something you'll always want on your body. My favorite tattoo is the one I drew for my grandfather after he passed away, it's special to me and makes me smile every time I look at it.
If tattoo's aren't your thing, consider getting a piercing. Still fun and rebellious but it doesn't have to be in your skin forever. I got my monroe done freshman year of college, but I took it out now that I am looking to get an internship. You can barley even tell I ever had it done.
Tattoo's and piercings aren't for everyone, but my advice is if you want one, get one. If you're worried about future jobs, get one in a spot where you can easily hide it. That's not hard, believe me. And lastly, one thing to really look into is your tattoo artist. Nobody wants a bad tattoo. Trust no one until you've seen their work. Trust me on that.