Do I need to do any course to be a fitness trainer?

Yes indeed! It would be highly unethical, even dangerous, for you to give exercise advice to someone without any qualification. There is a lot to be learned, including some anatomy, functions of muscles, the physical effect of exercise both generally and specifically.

The fact that you could even ask this question suggests a high level of ignorance about fitness instructions. There are usually at least three levels of qualification and you must study, practice and pass each before you can become a fitness instructor. Suppose someone followed your (unqualified) advice and then had a heart attack or pulled a muscle!