Need to earn some money and need it fast??

I'm on my brothers yahoo account but I am 12 years old and need to make some money. I am an experienced mechanic because I learned loads from my dad then I learned some here and there fixing up my older four-wheelers which weren't running when we purchased them. I was wondering if anyone had any suggestions of what I could do to get some money I can also mow lawns and that kinda stuff but it's not summer time yet so not many people need lawn care. I can pretty much do any physical work and I'm looking to earn about $1200 total withing a couple months.