When the coolant light is on, that means the coolant reservoir is low. Normally, you just need to add coolant to the reservoir until it reachs the MAX indictor. A coolant flush is necessary only if the remaining coolant is very acidic.
The coolant you buy from the store require mixture with water. Under normal condition, it is a 50/50 mix, but please check your vehicle's menu for recommendation. The store also sell premixed coolant, which you don't need to mix with water.
Before you open the coolant reservoir, make sure your car is cool. NEVER change coolant when your car is still warm, because the heat build up high pressure in the coolant reservoir, and when your open the cap, the hot acidic coolant liquid will launch out and land on your skin. YOU WILL GET BURN.