For the early US turns a lot is going to depend on your overall Allied strategy and what the UK is doing. To start the game the US doesn't have much to do in terms of combat but you want to make sure you buy the right units and start to move your forces into a more useful position.

Since there probably won't be much to do in combat and you will spend most of your turn backing up the other Allies, I'll go into a little more detail about some possible purchases.