Notice the Auto Resupply checkbox. If you activate it, your weapons will be fully loaded upon returning to the garage after a battle. As with repairs, if you lack the funds to resupply all weapons, no ammo will be purchased and you will have to enter this screen to do it manually. You will ALWAYS have to resupply a new robot manually, before entering a battle for the first time!
To the left of each weapon, you will find an icon indicating the currently selected ammo type. With the arrows on each side of the icon you can cycle through the different ammo types available. You can hover your cursor above the ammo type icon to see a description of the selected ammo's properties. Remember, energy weapons use no ammo and they draw on your battery instead.
Next to the Ammo Type icon is the Current Ammo slider. It shows the maximum ammo capacity of a weapon, as well as the amount of ammo you want to load it with.
By pressing the Max Load button on the top of your weapon list, all ammo sliders will be set to maximum so you can reload all of your weapons faster. On the right, you can see your Ammo Reserve count for each weapon. When resupplying, ammo will always be drawn from your reserve first. On the bottom, you can see the total cost of the ammo you have chosen to resupply with. Pressing the Buy button will confirm the purchase, should you have sufficient funds available.