Lay your tape measure against the wall at one end of your room, and then extend it to the opposite wall. This is your length
Do the same for the other two walls in the opposite direction. This is your width
If your room is L-shaped, measure it as two separate rooms. Measure the long portion of the L as one room and the short portion of the L as another room.
REMEMBER - To add on a minimum of 10cm width and length for carpet fitter