What should I do after completed courses and fastai book.
Try to explore certain competitions on Kaggle or even maybe participate in hackathons in the domains you liked studying in Part 1 of the course. Take up some projects that you find interesting online to brush up on the stuff you studied in the course.
Once you feel you are confident regarding what you have learnt, you can do the second part of the course or even try the NLP course on fast.ai (that is of course if you enjoyed the NLP section in the 1st part of the course).
Do I must review Python and PyTorch first ?, or I can do it while joining competitions.
You can work on the competitions alongside learning PyTorch and Python, which would help you review them as well.