Coolant

I am new to this car business and I have just purchased a 1996 Fiat Punto 1.1 55sx. It passed its M.O.T 3 weeks ago and it has all major work on it done.
However I took it for a drive on the motorway and it began to over heat so I pulled over and let the engine cool down. The same thing happened again when I was driving locally so I think it is something to do with the level of coolant in the engine.

BUT I DONT KNOW WHAT TO FILL THE COOLANT UP WITH IS IT JUST WATER OR IS IT SOMETHING ELSE! Please help me.