Yeesh. Did you buy a used copy of the game with no rule book? Anyways, I shall answer your questions, and all newbies are encouraged to use this board to ask other questions.
-Change into Mario by Walking into his dressing room as any character. The same is true for Luigi & Wario, except you have to go into their respective dressing rooms. To change into Yoshi, walk into the dressing room of the character you're playing as. To unlock the rest of the Bros., go into the paintings that have their pictures.
-To fly up, which seems to be part of your problem, Hold the down button in standard mode, or scroll (with your finger) down on other modes. You may be crashing into the ground because you are not high enough in the air to reach your destination. Also, you can fly farther by finring yourself out of cannons. On an off yopic note, the other characters can fly in VS. mode, which can be played alone.
If I was creative then you'd be laughing...