Unfortunately I did the same thing at that age. I had a boy's initial put on my arm and regretted it straight after. Luckily, my husband's name began with the same initial, lol, although I did have another tattoo put over it. Much more classy than a homemade job.
Like Ratter said, it's either laser or tattoo over it unfortunately. But there is a cream you can get to temporarily cover it. My friend used it when she was a bridesmaid. It's a bit like foundation and I think people who have scars and birthmarks use it.