You can craft Ammo using the F1 Menu, Press F1 > Mixtures > Ammo, then craft whatever Ammo you need, to craft ammo you'll need the ingredients that are listed in the crafting recipe, you also might need a workbench for this which can be placed in properties. To craft anything, I suggest using
http://calc.tinycoding.com/ . It's a great site to search for stuff like ammo, to which it will then show you what materials you need, how many you need, and how much it'll cost. You can even adjust the taxes!