I am given 20 ft by 18 ft by 8 ft for a room, but I don't know which number is which, like is 20ft the width or the length? I think this is a rectangular prism but I don't know what the values represent or what their order is.

Well the "width" or "length" is really dependent upon your point of view

Whatever....the floor dimensions are 20 x 18

And the height = 8 ft

And it is a rectangular prism....