Create a list of every item that you will need in your classroom. Include the quantity associated with each item, such as 25 desks.
Use a sketchbook to create a rough version of your classroom floor plan. It's a good idea to try several different approaches to arranging your floor plan until you arrive at one that works best. Hang on to all your sketches since you may want to change things around. You should make sure that you have the maximum space between rows to reduce tripping hazards.
Take your floor layout a step further by using a program like SketchUp or the online utility Classroom Architect. These let you convert your drawing into a diagram or a 3D model. You can even add desks and furnishings. This way, you can rearrange virtual furnishings until you are satisfied with your classroom's layout.