I was your age when i started. I packed 1 pushmower, 1 backpack blower, and 1 trimmer in the back of my bronco and did 4 different yards a day. Here is what I learned.
1.) Let them know your young and just trying to make an extra dollar while you are in school, they eat that kind of stuff up. You will have some customers be more receptive to that idea, than a pro, while others will want a pro
2.) Work your butt off in the neighborhood, neighbors will notice. I also use to put a professionally made sign on my truck while i was out on some jobs. I would get phone calls such as, "I was walking my dog and saw your sign and typed your number in my phone, I saw you working very hard and was seeing if you would mow my lawn"
3.) If your serious about doing this for a while, splurg a bit for some good equipment (RedMax, Husqvarna, Ybravo)