I had off and on bleeding/spotting for about 3-4 months after I got the Mirena. Since then it's been no problems (almost no periods) and I've had it about 4 years. I've been very happy with it.
I'm surprised though that your aunt and doctor recommended the Mirena. Was the Patch not working for you? Giving you side effects? Based on your profile it sounds like you don't have sex on a regular basis. Maybe go back and talk to your doctor. The Patch didn't work for me because it gave me awful headaches, but if you didn't have side effects then maybe that is a better option for you.
I'm not going to lecture you like some other posters have, but high school and ultimately college will be easier and less traumatic if you don't have to worry about unintended pregnancy and STDs. Sex is really a much better experience when you have an emotional attachment to someone and a trusted relationship with them. It sounds like you are a smart girl with a bright future.
ETA: PLEASE don't listen to the poster who suggested natural family planning and no birth control if you plan to have sex. That is fine for married or committed couples who can handle the responsibility of an unexpected pregnancy. I don't believe it's true that it is as effective as other methods. In fact, I can't remember the joke now, but there is a joke out there about women who practice natural family plan. The punch line is something to the effect of "they call them moms." Also, as a teenager your cycle might not be regular enough for natural family planning and you could ovulate at unexpected times.