Need guidance from seniors getting started with professional

Just cleared executive this june attempt, and I need help and guidance getting started with professional.

What should I know before getting into professional, like the ones already in professional can you guys tell what is that one thing that you wish you knew before starting prof, the mistakes that you made and any tips and guidance that there is and what helped you change your mindset from exe to prof.

What subjects should I go for while choosing electives?

Now that I cleared executive, do I have to register again? Then what's the procedure and when do I do it?

Would really appreciate if you helped the ones who cleared executive.