You need to complete orders on the merge board to get Coins. Orders are displayed on the top, and the items that are ready to be ordered will have a light green background. Use the Coins on the main screen to complete tasks, complete renovations, progress the story, and get extra rewards for completing milestones. If you go back to your merge board, don't forget to claim your rewards, they will also help you to complete more difficult orders. Continue to merge, and have fun!