Go to the shop if you want to pick up one of these upgrades:
Water Limit - You have quite a small water meter at first, and you'll definitely want to upgrade this as a priority. The more water you hold, the more land you can generate, and this will definitely help you on the way to a three star rating.
Lightning - Lightning's great for zapping clusters of enemies or volcanoes, but if you really want to let the lightning rip you'll want to upgrade its power here.
Lightning Gauge - Just like the Water Limit, upgrading this will give you a much bigger zapping meter! Useful for levels that contain plenty of volcanoes.
Top-Up Lightning - Getting tired of waiting for your lightning meter to recharge? Get an instant refill by spending your coins here.