High cholesterol at 17? Can someone explain my blood work results?

I'm a 17 year old girl and recently had a physical done. The blood work came back and I got high in ALT(65), high in cholesterol (227), high in LDL (131)... Normal in HDL. High in TSH (6.96), and high in thyroid peroxidase ab. I exercise about two times a week and I don't eat really badly for a 17 year old. I'm 5'4 and 115 lbs. What does high ALT mean? What's the difference between LDL and HDL? What does high TSH and thyroid perocidase ab mean? What could be wrong with me?