This may seem a daunting task but really is quite simple.
Treat each wall/floor space as a separate area. Measure the length of the wall followed by the height. These two measurements will determine the area you need to cover. (adjustments can be made for windows, furniture etc as required). Take the measurements and multiply together (length x height) 1.2m x 2.4m = 2.88m2. Move to the next wall and do exactly the same, once all walls done add each total area together to give total m2 you require for the walls. Now the enjoyable task of choosing your tiles.
It is always recommended to add 10% to cover cuts and wastage.
As always, we are here to help and if you are worried or concerned simply email or call us with your details of your room dimensions and we will advise you according.