Try doing what my daughter calls "seals." Lay on your tummy and raise your arms up, so you create an arch in your back. You can also do it with your legs and then both at the same time. Another exercise you can try at home is walking down the wall. Stand with your back to the wall and put your arms up touching the wall and "walk" them down. Don't try to go all the way to a bridge in the beginning---take it slow. Also for this one I would make sure you have someone spotting in case you slip(put some pillows down) and check with your parents about which wall to use!!!
As Graceful said, you won't get the flexibility quickly, but you will see improvement if you stick to a simple plan everyday.