These are just some main strategies (troop combos) for beginners.
• bombers + stugs + fighters (could be jets too). The bombers would take out all the ground while the fighters defend the bombers from other fighters. When the bombers kill all the ground units you would then send in transports (air usually but could use boat if coastal and you need to save troop space) and then the transports would let the stugs out onto the battle ground. Then the bombers and stubs would take out the turrets and then you'll finally take out the city.
•stugs + tanks + anti-air. This one is the easiest combo to get. You would send in tanks ahead of stubs and kill any ground troops while the anti-air takes out enemy aircraft (you won't always use the anti-air but it's better to be safe because if not then even a single bomber could kill your whole army) all the while the stugs would be shooting at the turrets. You would then attack the turrets with your tanks too then finally the city.
•Naval + ground and/or air. This one is simple. You bring in any ships that can shoot (destroyer, Cleveland cruiser, North Carolina battleship, and the Bismarck) and destroy all that you could with them. Depending on what you feel like (and have) you could use either/both the aircraft carrier and the transport boat to send your clean up crew in.
•all air. This one is the simplest one. You use bombers to take out ground troops and turrets while fighters sick around and shoot at any planes trying to stop you.
•artillery + stugs. In this one you use artillery to take out what you can from afar then send in stugs as a clean up crew. You can add in tanks to soak up damage for the stugs.
If I missed one or you have another strategy then post it in the comments then I'll add it on to the blog with your credit.