Take a photo of one to the paint department and ask for their advice. You should buy the spray paint made just for plastics.
Great answer from Fiddledd, also be sure your chairs are very clean and dry before you paint.
How many chairs are you planning to paint?
How much area the product should cover will often be listed directly on the can! Good luck :)
I painted four chairs (medium size) and got 2 chairs per can.