Your car has 4 tire pressure sensors, one per tire, and when they detect that your tires are getting low on air, there is a symbol that will show up on your dashboard. It’s sort of an upside down horseshoe with an exclamation point in it:
Most gas stations offer free or very cheap compressed air. Each car has a different needed tire pressure, some machines tell you how many pounds your tire requires, and blink or beep once you’ve reached the fill point. If the machine doesn’t tell you how much air you need, you can find out by opening your door and either looking in your door jamb or on the side of the door. It will tell you two pressures, the hot psi and the cold psi. The maximum amount that you should fill your tire to is the listed cold psi so it doesn’t get too pressurized. In the heat your tires may show that their psi is higher than that, as air is a gas and it expands in heat and contracts in the cold. It is best to check tire pressure in the morning, before the days heat can get to it, so it has the proper cold psi pressure.
Once you know how much air your tire needs, all you do is unscrew the air chuck and put the nozzle around the metal part and pull down the lever until the tire reaches it’s desired pressure. Don’t forget to screw the cap back on so you’re not losing air everywhere you go!
Don’t forget to check if the rear and front wheel pressure has to be different so you don’t overfill or underfill!