You can open the inventory by pressing I or TAB. You will find there a list of items from your backpack, then a depiction of the avatar and the stuff it's wearing. The last thing is the section with weapons that you're wielding. You can simply drag'n'drop items from the inventory into the usable items fields - same goes for disposing of stuff you don't need.
Check out the slots underneath the firearms - there you can put different augmentations that will improve your aiming, accuracy or increase the ammo capacity of the gun. It's a sort of very rudimentary crafting mechanic. Weapon mods can be found in different parts of the map - collect them, because in case of long-range weapons more accuracy or more ammo capacity can mean the difference between life and death.