To determine the headlight bulb you need for your vehicle, you can use the bulb finder tool provided by various websites such as Sylvania Automotive, AutoZone, PowerBulbs UK, NAPA Know How Blog, and Bulbs.com. These tools will help you find the correct replacement bulb for your vehicle by inputting details such as the year, make, and model of your car. Once you have found the correct bulb, it is recommended to replace both bulbs at the same time to ensure equal brightness and color.