Well, in general, nothing wrong with tat's. I have two and would like a couple more.
But, for a personal opinion, i don't care for tat's on women. Tat's look masculine to me, even 'girlie' tat's.
The key to getting tat's is take time to think about what you want. Figure out what you want, how big, color or black and white, and all the details. Research tattoo parlors and make sure to find one with a good reputation. This way you not only get a good tat, but don't have to worry about how clean the place is. Make an appointment, don't be a 'walk in'. So just don't get in a hurry. When i was 18 i got in a hurry, picked a place out of the phone book, and drove over. He was in a hurry, since i was a walk in, and he didn't do a good job. And because i was rushed, i didn't make all the right choices, either.
Far as the issue with your mother, its tricky. You're technically 18, an adult, but at the same time you are under her roof and this is apparently important to her.
Also, keep in mind tat's aren't cheap. Even small ones cost more than you may expect. If they start the tat and within a few seconds they change their mind, they will still charge you. Could be $50, or more even if you don't finish. The back is a difficult area to sit through, so be positive you can handle it.