If your car doesn't have regular coolant leaking problem, you can basically buy a small bootle of coolant and refill in your car,s coolant reservor; or, if your car,s next maintenance is coming very soon, you can ask garage to top up it for you...
If you go to store to buy coolant, make sure you check your car,s service manual to get the correct type for your car... If you are not getting pre-mixed type, please mix coolant with certain percent of water by following coolant,s instructions...