You can find ghasts in the Nether. To create a nether portal, you need 14 obsidian. Make a 4x5 portal, and then activate the portal with flint and stone. Now enter the portal with a bow and some arrows. Wait for a ghast (large white square) to appear. Then simply hold your bow ready to shoot. Once he starts shooting fireballs at you release an arrow to hit the fireball back at the ghast.