I am going to put a carpet on a room. This room is on a ground floor and it's actualy cold (I can feel the cold on the ground).
Is there some material that I can put between the carpet and the floor to provide better isolation?
When you install carpet you're first supposed to put down a carpet padding. This is available in a number of different materials including urethane foam and rubber though most commonly these days I see a recycled urethane product. It's main purpose is to prolong the life of the carpet by absorbing some of the shock of being walked on. It also provides a softer feel underfoot as well as some insulation.
If you want to increase the R value of the carpet padding, choose the densest and thickest padding recommended by the manufacturer for the carpet you're buying.
The Carpet Cushion Council (apparently it's a thing) has some example R-values for carpet padding listed on its website.