As you progress, you will come across stages where you have to shoot IEDs before they blow up and do serious damage to your vehicle. You will recognize the IEDs by the red target that appears around them. Aim and shoot at these before you shoot at anything else. Sometimes you’ll see them around when you’re stationary, too. If you see them while they’re not moving, then shoot them when there’s a crowd of zombies around, and they will blow up the zombies instantly.
Don’t just upgrade the vehicle and the weapon. Upgrade the AT-4 Rocket too, because when you’re in a sticky situation, an upgraded AT-4 rocket can do serious damage to enemy vehicles, and can blow up a huge quantity of zombies. Upgrade the ammo cap for more shots, the damage to make it more effective against vehicles and bosses, and the damage radius to take out more zombies at a time.
If you desperately need to rack up the ZMoney or the BX, go to missions that you have played before that you know you can beat, and play them again and again. Losing on a mission puts your vehicle out of commission for awhile, but if you replay a winnable mission, you can play again and again and keep killing zombies and earning money.
Keep completing missions and eventually you will be able to unlock more guns besides just the standard 50 Cal and the AT-4 Rocket. If you want to see how a gun works before you equip it, tap on the demo button to watch a quick demonstration. The riot gun is a giant shotgun, the minigun is like a 50 cal on steroids, and the smart gun automatically aims at zombies and vehicles, taking almost all of the aiming work out of it for you.