Every weapon has an Ammo type associated with it. You can keep track on the Inventory Menu display. Ammo does not consume Inventory space, but there is a limit on how much Ammo of each type you can carry.
Ammo is found in the world like all other game Items.
Weapons and the Ammo they use play a big role in how far and how fast the bullet will travel.
Watch the tracers that come in most Ammo types to learn how to compensate for ballistics over long ranges. You may have to slightly lead a moving target and will often have to aim higher.
Although still very much a work in progress for this Beta, and not final, the Scopes and the Optics within them can help you compensate.