Make sure you wear gloves and tie your hair back carefully. Leave someone else's hair at the scene, maybe some school bullies or something. that'll get the heat off of you.
Go to a Circle K or 7-Eleven, do NOT hit an indy store, watch and see if your local Circle K's have security guard, too. Scope out a good store and a good time.
Get yourself some shoes that add 6 inches to a foot of height, and one of those fake-hair hats in a different color. Dark sunglasses, too. Come back at that good time, check and make sure you're clear, you gotta be careful here anyone around could really ruin your day. Pull your gun and demand the register be emptied.
The clerk WILL hit the silent alarm, so be ready to flee, and fast. But they should give you the money, if they stall too long, book it anyways.
Remember that persons hair you left at the prior house, leave more at the store.
Then you just need to enjoy the .. $87 that you got, and risked going to juvie for. :)
Step 3: PROFIT!