After completing the tutorial you will go to the main menu where you select which campaign to play. You should start with campaign 'Inception', which is a short campaign that should be fairly quick to complete.
Some campaigns must be purchased for a small price before you can play them. When this is the case you will be able to play a demo of the campaign first. If you have previously purchased the campaign, hit the 'Restore Purchases' button available on all purchasable campaign pages.