To make a splash potion, you need to:
1. Go to the nether, kill some blaze, pick up some nether warts
2. Make a brewing stand out of 3 cobblestone and 1 blaze rod
3. Make a glass bottle (from glass blocks) and fill it with water
4. Add blaze powder (made from blaze rods) to the fuel section of the brewing stand
5. Add nether wart to the bottle of water to make an awkward potion
6. Add gunpowder (from creepers) to make it a splash potion
7. Add a spider eye (from spiders) to make it a splash potion of poison
8. At night, look around for a witch and throw the poison at her
There you have it!