To make two sections of floor level to each other, you have several options. One option is to fill the lower section with self leveling concrete but be careful that the use of that material has thickness limits and after that limit is will give you trouble during installation. Another option is to use plywood but make sure that your construction type allows this installation. There are other alternatives too based on your specific circumstances. Check with a construction professional first, this can seem like a simple task but it is not.