Set up a table in front of a popular place that lots of people go to. Make sure to bake a lot. I wouldn't charge more than $.50 per cupcake depending on the size. people won't buy if they're too expensive.
Bake a large supply of cupcake, the set up a stand using a table and a couple chairs (preferably in a place with a lot of people. for example, parks, outside supermarkets, outside schoolyards, or right outside your home :) ). Charge 50 cents, 75 cents, or a dollar for the cupcake depending on how much toppings there are and how large it is. You should also have an adult by your side to watch over you and help you. Have fun! :)
You could sell them like you'd sell lemonade, get a table, a sign, maybe some balloons to draw attention. Or sell them outside a store. The bad thing is you'd probably have to have a permit to legally sell them. I think it would be bad to sell cupcakes that you bought at a store that says: not for resale on it cause you could get in trouble if you get caught selling them maybe. Might have to make home made ones.