There are various options in conditional formating. Some of the conditions are predefined and can select the appropriate one from the options provided.

However the Select "New Rule" from the options,
Select " Use a formula to determine with cells to be formated"
Enter to formula in the area provided ( Eg: "=< 1000" ). and the kind of formating which is required( Eg: select a Red col. to fill the cell)

ITS EASY - TRY IT !!