Determine how many of them you want to use. The size and design of your finished quilt will depend on the amount of material you have available.
14x14 inch (35.5cm) squares are a common and comfortable size to use, but you can enlarge that to 18x18 inch (45cm) if the shirts are XXL and shrink that to 10x10 (25cm) or less if you will be using children's shirts.
Common blanket sizes are:
Crib - 42" x 72" (3x4 or 3x5 shirt grid = 12 to 15 shirts)
Twin - 66" x 96" (5x8 or 6x9 grid = 40 to 54 shirts)
Double/full - 81" x 96" (6x8 or 7x9 grid = 48 to 63 shirts)
Queen - 90" x 102" (8x9 or 9x10 grid = 72 to 90 shirts)
Standard King - 108" x 102" (10x10 or 10x11 grid = 100 to 110 shirts)
California King - 102" x 110" (10x11 or 11x11 grid = 110 to 121 shirts)
Note: you can use "sashing" or strips of cloth between the T-shirt panels to reduce the number of shirts needed... these numbers are approximate, and are for a quilt top made of nothing but tees, with no sashing.
Lay the tee shirt out flat. You may want to iron the shirts (note that many designs on t-shirts have transfers that may melt, so test a small area prior to ironing) to get out any wrinkles still left after washing and drying.