Measure the dimensions (the distance from two of the walls to their opposite walls) in metres, then multiply the two measurements by each other ie. if one dimension is 3 metres, the other is 4 metres, your room is 12 square metres.
You'll probably get quite a lot of flak off people on here for this kind of question.
Remember measurements in metres squared are different from measurements in square metres. Some architects don't, however, know the difference but it could be important.