Firstly, I agree with WBahn's response regarding the approach you should take with the Nand2Tetris course. I would encourage you to try and figure things out for yourself and not deprive yourself of what Nobel prize physicist Richard Feynman describes as "the pleasure of finding things out".
In your post you indicated that you would like a career in computer science, a very broad field these days. You identify yourself as a "beginner", but that is really too vague a characterization to provide any solid advice in pursuing your goal. For example, do you have any background at all in computer programming using a modern language such as C++, Java, or Python? Such knowledge, probably at a little above beginner level, will be needed to complete the course project in the sixth lesson.
I have taught introductory computer science in high school (10th thru 12th grade) for the last few years as a volunteer instructor in the national TEALS program. My students are usually absolute beginners; they have no experience with computers other than as end users. The Nand2Tetris course would certainly not be a starting point for someone at this level. I think the book, while excellent, assumes that the reader has a basic understanding of at least how programming works.
If the above describes your current level, I would be happy to suggest other courses available on both the Coursera and edX platforms which might be a more appropriate starting point.