Acquire new items by looting defeated enemies.
A beam of light shining from a downed enemy means that they have loot. Right-click on the body to open the loot window.
You can right-click each item in the loot window to take it. However, in most cases, it's easier to click on Take All to grab all of the items in the window.
Different-colored beams and item names correspond to item quality.
If you see a green beam, be sure to get the loot! Even if it's not useful to you, it's probably valuable and worth taking to sell later.