Did you used to read Seventeen magazine when you were young?
Back in 7th grade, I had an issue of Seventeen (not this one - I'm not THAT old!) that showed you how to make your dad's tshirts into a minidress. I wore that homemade, oversized, white thing everywhere and was so proud that I'd made it myself! Here's a quick tutorial for a teen sized dress, updated for today's style.
You'll need two shirts. The main shirt should fit you according to your taste. The other shirt should be your dad's size - BIG.
Level off the bottom of the tshirt that's going to fit as the bodice.
Cut off the dad's shirt below the armpits. This fabric will form your skirt.
The lower half of dad's shirt should be wider than your own shirt.
Gather the skirt to roughly the width of your fitted tshirt bottom.
Pin in place (good sides together). This section will have to stretch around your hips, so as you stitch stretch the fabric gently.
Add some trim if you like. This dress is going to be a costume for the stage in our basement, so I picked music notes.