As prey you want to survive and evolve as much as you can. You get evolution points from greenish-blue pools around the map. (-pools to have the pools pinged on your mini-map) Once you have enough for a higher evolution, you purchase it like you would a unit. Note: your evolution points RESET when you die or when you evolve. Now if you are in a later game as prey, and have the final evolution team up with your fellow prey and try to kill the predators.
If you decide to play as the predators, you get evolution points differently than the prey. You obtain evolution points from either killing a prey, or killing critters around the map. Your goal is to kill prey as quickly as you can so that you can earn some fast evolution points. (Predators are not recommended to newer players)