You can get this the first time you arrive at your safehouse, which is the first thing you are required to do after the prologue. Just walk over to your workbench and enter it. Here you can craft many things, such as bullets, grenades, and first aid kids. You will have to use the workbench to be able to complete the game, as you will eventually run out of bullets. Just make sure you are collecting materials by looting dead bodies, dead animals, and opening all of the crates you see laying around.