How Much Should I Get Pay?

O.k so here is the deal.I am 12 going to be 13 in the summer.So my mom's mechanic offered me a job to help him out.Butwhat I am wondering is how much should I get paid.Should I get paid by month,every week.every 2 or 3 weeks?I will be doing everything a normal mechanic does I will be learning new things to help my mom out since I live with only my mother and 1 brother and 1 sister.So if you can plpease tell me how much to tell him to pay me.By the way the mechanic is a friend of ours.I am doing this job so I can be "distracted" over my summer.So that I wont just be at home sitting around wasting my life watching t.v.Any help will be appreciated.

10 POINTS FOR MOST REASONABLE ANSWERS THANK YOU

I think it'd be best if you and the mechanic sit down and talk about it, negotiate it between yourselves--what he is comfortable paying you, and what you are willing to accept. Weekly pay wouldn't be a bad thing, but see what his pay schedule is like--he can best afford to pay you if he has his money first!
But you are going to be doing a job; try to be as professional as you can, and you'll both be happier for it. Don't take it any less seriously just because you see it as a distraction. You're both helping each other. If you make yourself helpful and dependable, you increase your chances of being asked back; and these are useful skills and work habits you'll be picking up. I hope you'll do well with it!

If I were you, I'ld be happy with whatever he paid me. After all you are not legally supposed to work yet.