Maybe start with a costume that is a mix of found / purchased pieces and made pieces. For example - Maka from Soul Eater:
She's the one in the plaid skirt and the black coat. The basic pieces are:
A black coat - you can purchase something similar and alter it to tuck in the waist. It's a good lesson on how clothing is constructed. You could add the over-sized cuffs and button detail.
A plaid school girl skirt - an easy find
A yellow sweater vest - also relatively easy to find
White gloves - easy to find and inexpensive
A white button down shirt - who doesn't already have one?
A green striped tie - easy to make
Giant clunky boots - you can take some black boots and add the detail with white fabric or foam or some other material.
The local Salvation Army / Thrift store is your best bet for finding these items for not a lot of money. Spending a few dollars on a complete piece can sometimes be worth the frustration of making something from scratch. There is plenty of detail to be added in a costume like this - buttons, trims, proportions, hair, makeup, props etc - all that go into creating a successful cosplay. You also get some beginner sewing experience.
You can break down a lot of costumes the same way. I'd be happy to help you walk through one if you want.