18' by 15' simply means 18 feet by 15 feet.
simply convert feet into yards.
1 yard = 3 feet
18 feet * 1yd/3ft = 6 yards
15 feet * 1yd/3ft = 5 yards
Get the area in square yards:
6 yards x 5 yards = 30 square yards.
You should order 30 square yards of linoleum to cover your 18' by 15' kitchen.