Every mission you play earns Coins, to progress you will need to invest these Coins in Tower Upgrades which can be found by clicking the Upgrade button on the main map screen found in the lower right hand corner. These upgrades allow you to access stronger towers in levels, these towers are key to your victory in later levels!
To boost your defence be sure to use Reinforcements and Airstrikes, if you are lacking Infantry or are out of Tesla charges then they can save the day!