Before starting a game, you need to buy or select the guns you plan on using as well as the ammo. Click on the gun button at the bottom to get the gun selection screen to come up

When your gun selection screen come up , you will have vast array of weaponry to use against your zombies.

After you decide which gun you will use, be to click buy then click select to select the gun

Be sure to pay attention to the ammo the gun needs and make sure you have enough in the ammo screen before you select a map and enter a round.!