Keeping Victor alive is your top priority. Your health bar (green) depletes when you take damage. If it reaches zero, you'll be 'Wasted' and respawn at the nearest hospital, losing some money and all your weapons. Armor (blue bar) acts as a secondary health pool, absorbing damage before your health starts to drop. You can replenish health by picking up health packs (red cross icons), eating at fast-food restaurants, or visiting prostitutes. Armor can be purchased from Ammu-Nation or found as pickups. Always prioritize picking up armor before a major confrontation, as it significantly increases your survivability. Don't be afraid to retreat and find health if you're low during a mission.
