Merchant of the Skies

The game will prompt you early on to buy an island where you can harvest Sand but it’s a better idea to buy one where you can harvest Wood instead as it’s more valuable and more needed early game.

When you’re on an island hit move one menu item to the left, click on it and you should be offered the chance to buy the island. You should probably only spend 50% of your max cash this early in the game. Anyway, once you do this achievement will pop, even before you’ve built any buildings on it.