Job switching advice

Hi! I was placed at Cognizant and received my offer letter, but I am still waiting for my joining letter. It has been six months since I got the offer.

I don't want to waste this time, so I want to use it to improve my skills. My goal is to build a strong career and prepare for better opportunities in the future.

Could you please guide me on what topics I should focus on? Which skills should I learn now that will help me both in my first job and when I switch to a better company later? Any roadmap or advice would be greatly appreciated.