To echo many others in this thread, the very first thing to do, before any sewing, is get a good stabilizer on the back of the tee shirt. I've seen Pellon brand recommended, but i find it doesn't seem to adhere well in Houston humidity, so I've switched to June Tailor brand.
Be careful ironing or sewing on the front of tees. Large rubberized designs can melt onto your iron or snag your presser foot. Iron or or sew around those, never through.