Identifying which commands need shortcuts
Obviously, you can set shortcuts for whatever commands you want. In fact, you could probably create a shortcut for every single command you use. This might now, however, be the ideal way to spend your time in SketchUp and probably would be difficult to remember. Shortcuts that you cannot remember will not get used and not end up helping you to be better at SketchUp.
What you need to do at this point is identify the commands that you use most often and make sure that they have shortcuts assigned to them. After that, we need to look at the commands that you plan to learn and hope to use more often and get some shortcuts applied to them.
Assuming that you are reading this book chronologically and not skipping anything, you should have a pretty good idea of what those commands are from Chapter 6, Knowing What You Need Out of SketchUp. If you skipped that chapter or were just so excited about shortcuts that you jumped right to this chapter...