After deciding the type, shape and size of deck you'll build, the next step is to estimate the decking materials needed. If you use a ready-made design, and the materials list is provided, this work is already done for you. But if you design your own deck, or use a variation from a standard plan, you'll have to estimate material requirements. In estimating, it's better to overestimate since you can always use any excess material in other projects, such as benches or planter boxes.
There is nothing worse than running out of material and dashing off to the timber yard before they close (hopefully) right in the middle of the project. (On the other hand, you may just be looking for a break!)