Don, the easiest way is to start by measuring the area in feet and decimal parts of a foot. For example, 6' 6" would be 6.5' and 8' 9" would be 8.75". You also have to measure the depth of the slab you want. This is normally around 4" (.333') or 6" (.5'). Now multiply the length times the width times the depth. This will give you cubic feet. Add 10% (multiply by 1.10). When you have this number, divide it by 27, the amount of cubic feet in a cubic yard. Always round up. Now you have the amount of cubic yards of concrete you need to order for your project.
If you are pouring a slab, don't forget rebar, rewire, or to order fiberglass filled concret which will hold the concrete together and prevent cracking. If you are pouring a big slab or next to an existing building, you will want to add expansion strips if you are going more than 8' in any direction or next to the building.