Select your tool by clicking on it or using the number keys and click (and possibly drag) to use it. Be strategic though since each takes a separate amount of time to reload.
You're all we've got to defend the paths from enemies. You have multiple tools to take down these tanks, cars, and whatever else they can through out you. The twist is instead of having turrets to do it like a tower defense game you have air support that you control. You have cannons, morters, bullet sprays and more at your disposal as you progress through the game.