when i was in middle school, i made my own bussiness cutting grass with a 20" push mower. it sucked because the only time i had to do it was weekends durring the school year, but i had plenty of money to spend at that age. 50 bucks for front and backyards. i didnt realize when i printing out add that some people had enourmous yards. but those same people were consistant customers so i always had clients after building a good relationship
so far you have not said your age. my daughter does odd jobs around the neighborhood, she mows lawns in summer, rakes leaves for folks in the fall, and does side walks in the winter. she is 14 now but has done it for several years. do extra for mom and dad, make a pact, you do the garage, you get X amount of money, etc. works for my kids.
i would hate to do my garage (its a 2 story pole barn converted into a repair shop) and is infested with bats we have tried everything but cant get the little winged rats out........ and as of right now is packed with extra junk (my dad is a bonified pack rat) here a list of shops etc in my area that i could work for when the oppurtunity comes: general store (great shot considering they know me very well and im their best customer), library (though i highly doubt it), donkey farm across the street(he always needs a couple hands to help out since his wife has become handicaped, i might get a shot out of local fire hall straightening up or i might get a shot out of blacksmith yes i said blacksmith
I did yardwork, raked leaves, babysat, cleaned house (some people loved my combined babysitting/housecleaning service, I funded a cross country trip after high school that way), petsat, sometimes volunteer work at the senior center paid off with contacts, elderly people wanting help with odd jobs...looks around, ask around, post flyers in the grocery stores, something will work out
you could always donate blood and sperm for cash. depending on how old you are...but thats the lazy way out.
I would highly suggest steering clear of pet shops....unless you know the people really well....had a bad experience with one of the little shops in my area because i refused to make the store an extra buck by selling people huge fish for their small aquariums and was accused of "wasting store resources" for giving the little furry animals food and water....yeah it was overall a bad experience lol not there anymore thank god.
anyway, just be sure you want to work at a pet shop before you work there, bad stuff happens in some of them