You can get your health (life) back by going to a restaurant and eating, getting a drink from a soda machine, or sleeping (going to your safe house and saving the game) or by finding health packs in certain missions. The health packs look like small medical kits with a cross on them and have a green glow around them. Either of these would refill your health to the max.
As for armor, you cannot just find armor in random places. In some of the missions, people will hide them for you to pick up and other times people you kill will drop them. Also, you can go to a gun shop and pick up the armor. Armor looks like a vest that is on the ground and has a red glow.

Keep in mind, neither of them float around like the armor did in San Andreas, so they can be a little harder to find.