Your car’s coolant/antifreeze reservoir should be easy to identify by its coloured lid, with many cars having a yellow coolant/antifreeze cap. You’ll probably find that there’s a warning symbol on the cap too, which highlights the danger of unscrewing the cap when the engine is running or hot.
The reservoir itself is usually made of thick, semi-translucent white plastic, and you should be able to see the coloured coolant/antifreeze (often pink, yellow, or green) through the side of the tank.
If you’re struggling to find your car’s coolant reservoir, the best thing to do is refer to the owner’s manual, which will have a diagram showing different parts and components under the bonnet. If you no longer have the manual, head online and visit the manufacturer’s website or forums dedicated to the make and model of your car — a fellow motorist should be able to point you in the direction of the coolant reservoir for your specific type of car.