Damage received while on-foot (from combat, hostile fauna or environmental damage) will first deplete your shield.
Your shield will recharge over time. Take cover and avoid taking damage while the shield is low. Damage taken while shields are down will damage your health.
Purchase plans for a Personal Forcefield at Technology Merchants. This is a powerful combat shield deployed from the Multi-Tool.
Exosuit defensive and life support systems can be upgraded with new technology. Visit technology merchants on space stations to upgrade.