At the bottom of the screen there are 5 buckets, with each bubble you shoot they are filled a little more. Until these buckets are full, when that is the case, the next bubble you shoot is worth double points. These same buckets give you extra points when the bubbles that you shoot loose fall in. The bubbles first bounce against fishes which gives you even more points.
When a bubble falls into a bucket you also receive 1 coin. You can see the total number of coins at the top left of the screen. If you press the plus here you can watch a short video for even more coins (up to 100 pieces!). This ensures that you quickly collect coins. With these coins you can then purchase nice upgrades. These upgrades are: