I'm 14 and want to start doing weights? Help?

Yep, you are skinny. You need to eat more proteins and put on some weight as you exercise. Muscles weigh more than fat and to build them up you need to start slowly and use light weights. Increase the reps until you're confident you can do them easily. Then increase the weights and start over with few reps and increase slowly. There should be some instructions with the machine. Follow them carefully, I estimate your weight at 98 lbs. Your average weight should be about 115-125 depending on your bone structure. Good luck and stay healthy.