One of the easiest methods for estimating how many tiles you need is to measure the height of the wall and the height of your tiles.
You can then estimate how many tiles will be needed to go up the wall <count half tiles as full ones>.
Do the same for the wigth of the wall , then multiply the numbers. This will giveyou the number of tiles needed to tile the wall you have measured .
Work your way around the room and adjust the amount for windows, doorways.