Touch the left half of the screen to move your character back and forth. The right half of the screen is divided into a "shoot" area where you tap to shoot, and a "jump" area where you tap to jump. (You can change the control setup through the in-game Settings menu.)
Every time you grab a green portal, you'll switch characters. Every time you grab a purple portal, you'll add a new character who trails behind you. Shoot all the ghosts you see, and grab the coins to buy upgrades, power-ups and new costumes. Get three stars on each level to earn bonus coins.