HELP! WITH COVER LETTER!?

Okay I'm sixteen years old, and am on leave from school as I have finished everything. I'm looking for a part time job, so I can make some money while I'm at college as they have stopped ema, and I really need to save up some money for, car (next year, driving lessons, insurance BLAH - BLAH. I am applying for a Sales co - worker job in IKEA, and they are asking for a cover letter, only problem is I don't know what it is, or what to right. As I have no previous experience (except a paper run). So can anyone help me?