When we started this book, in Chapter 2, Exploring Robot Building Blocks - Code and Electronics, we first learned how to look at robots as a sketch. I suggested that you make quick drawings, as rough as you such as, with a pen, and then move on to more formal block and layout diagrams later.
Every robot starts with a bit of inspiration. Perhaps there is a competition you want to try, maybe you've seen something like another robot or an animal you want to mimic (crabs are fascinating!). Other inspirations may come from seeing an amazing new part or sensor, or wanting to learn/play with a new skill. You may even have made a list of amazing robots you want to try and build.
Before building a robot, make a short bullet-point list of what it will do, what sensors/outputs it will have, and what it might have to deal with. This lets you focus your efforts...