The PT-19 is the best flying plastic trainer ever made. Try to talk to someone in a hobby shop, though, and see if you can get an adult to help you fly it. Two problems:

1. You will crash it - no if's, and's, or but's. Crashing on asphalt will probably break it beyond repair. On the other hand, you may be able to crash many times - over grass - with little damage. The object is to try to make it last until you can fly it without crashing.
2. Taking off from grass will be difficult, the wheels will not want to roll. The best place is a baseball infield. I would not use the cardboard idea untill you've learned to fly. Trying to take off from cardboard will have you pulling up on the handle right away. As soon as it's airborne, you'll be overcontrolling it and crash. You need to take off from a site that will let it fly while your wrist and hand DO NOT MOVE.

If you get serious, buy the Sig SkyRay as recommended by many here. The solid wing makes it easy to put together, and makes it almost indestructable over grass. At the same time, the light balsa construction will let if fly much better than any plastic plane. A Cox Golden Bee or Black Widow is best. Keep the PT-19 as is.