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Look around at all the floor that will need to be covered. This includes everything bordered by walls, but also less obvious places like the floor inside of closets. Sketch the floorspace out on a sheet of paper for reference. Run a tape measure down one side of the room to get its length. Move the tape measure and record the other wall in the same way. Write these measurements down on the sketch you made for reference. If there aren't any obstructions or unusual aspects to the room, the length and width will be enough to calculate the area. Take the length and multiply it by the width to get the area of floor space in square units. For instance, if one wall is 10 feet (3.0m) and the other is 8 feet (2.4m), multiply these to get a total floor space area of 80 feet (24m) square. If there are any closets, obstructions, or angled areas in the room, you'll start with this basic area and adjust it with a few more calculations to get the actual total amount of floor space. If you have a simple room with no obstructions or unusual shapes, look up an online floor space calculator. Enter the length and width measurements, and the calculator will compute the area.
Map out the entire floorspace. Measure the length and width of the room. Multiply to get the area. Use an online calculator for a quick solution.