Depends on how you want to finish it. If you want to quilt it, then you will need to have your top (tshirts) batting (middle) and fleece(backing). With fleece and a tshirt top, I would use the lightest batting I could find as it will be heavy enough as is. Then when you get the layers together, you would quilt them and bind the edges in the normal way.
If you dont want to do batting, I would put the right side of top to the right side of backing and sew with 1/4 seam allowance all the way around, leaving a spot open to turn, then you can knot it, like a comforter to secure the back and top together. Also might want to check your local library for books on tshirt quilts or take the top into a local sewing/quilt shop for advice as well. Good Luck!!