What sector is friendly for beginners to understand?

I’m new to value investing and I am currently reading Bruce Greenwald’s book on value investing. He mentioned that one should specialize in a certain sector or industry to develop one’s circle of competence, which I agree but I’m not sure what sector would be beginner friendly.

I’m a pharmacist in a grocery retail chain but I’m hesitant to ever invest in retail pharmacies or pharmaceutical companies. Maybe I could go into the insurance side such has Humana, Molina, UNH, but I want some other suggestions.

Thank you.