The Video that is included is by Minecraft Tutorials so check him out
Step 1: Items You Need
13 Green wool
8 Pink wool
8 redstone repeaters
7 redstone dust
1 pressure plate
1 water bucket
1 sticky piston
Step 2: Start Building
1. Place a fence post with a wooden pressure plate on top. Buildup 3 blocks on the right, then 2 blocks on the left with a dispenser on top, facing toward the 3-block tower.
2. Lay a row of 4 blocks on the ground, heading away from the pressure plate. The TNT will be fired from the pressure plate and the water will sit along the row of blocks.
3. On the right of the water bed, but 1 block higher, place a row of 4 dispensers inward. Mirror this with a row of 4 blocks on the left. Remember, you’ll need to hold down shift to place floating dispensers next to one another in the PC/mac Edition.
4. Create a pink timing circuit. This is a 3x3 square with a hole in the middle and a block missing from the corner closest to the dispensers.
5. Place redstone dust along top of dispensers. Then, in the corner of the pink circuit where the block is missing, place a sticky piston facing upwards, with a pink block on top. Place 4 repeaters set to 4 tick of delay on top of the pink blocks in an x formation. Connect them with redstone dust.
Step 3: Buildin Con.
6. Add a chain of repeaters leading from the pink circuit to the dispenser at the front. Make sure they are all set to 4 tick of delay so the TNT can travel a decent distance before exploding.
7. Fill the central water bed with water. Now place a button on the side of one of the dispensers (any one will do; just remember to hold down shift if playing on PC/Mac edition) and fill all five dispenser with TNT.
Step 4: Time to Test
Your cannon is now ready to give it a try