Its only a drop. The best ways to get it are:
1. Set up a mob farm for creepers (You are noob so do this one, its fairly simple and somewhat basic
2. Run around killing creepers with a looting sword... good luck.
3. Make a witch farm. (Very advanced and takes tens of hours)
It only comes in drop form. Creepers, Witches, and Ghasts all drop the stuff. The best way to get Gunpowder would be to set up either a Creeper farm or a Witch Farm. Creeper farms are easier, but with Witch farms, you get all the other stuff Witches drop. (And they can drop a lot of different items!) Ghast drops are notoriously hard to pick up, so don't go for those.