Much of what you will want to do in life is protected by a barrier that only allows qualified and quality candidates to come in. The "gate" through that barrier is known as the recommendation letter. Here is a link to some really good information for 9th, 10th and 11th grade students to read and live out BEFORE asking for a letter from a teacher in the senior year.
http://grownandflown.com/letter-of-recommendation-teens-need-to-know/