Computer Science is a very broad topic. Most of these series are information and theory based courses, there are practical elements with every course but many lessons will be discussing an area of interest and exploring the concepts and differences between them. The computer science elements covered in these series are usually seen as specialist topics and look to broaden any learners understanding.
Computer Science, in it's simplest terms, is the study of computers. In reality, it becomes a very complex and intricate topic to discuss. The vast complexity of computers means that computer scientists often specalise in a chosen area. Some of the most common specalisms are hardware (the physical components of a computer) and software (the programs and applications that run on a computer) but even these have large subsets of specalisms such as computation, architecture, data processing, databases, networking, security, the list goes on.
All computer scientists have a common goal; to enhance, increase and improve a computer (in their specalism) to work reliably for a given need or goal. In almost every circumstance, specalisms are linked, so an improvement or modification in one area will effect another (or at least mean improvements can be made).
After watching the video and reading the details above, it is time to choose a topic. There is no best choice to be made, as you have seen, it depends entirely on your own thoughts and which area interests you the most. Once started, you can always change, or restart a topic at any time. There is no restriction to skipping guides or re-visiting pages for missed information. You could also opt to study all at the same time (although a better approach would usually be to complete one after the other).