How ı can determine my spesific area?

Hello everyone. I'm studying mechanical engineering but now I'm at Foreign language preparation class. I am improving myself about aircraft and space areas. I wanted to be expert in these areas. I am looking some books. Such as "Aircraft Design" (Daniel P. Raymer) and I am looking Aerodynamic. I ve talked this issue with someone and they said "You have to determine main, spesific, area. Yea, you want to be expert in these areas but which one? prupolsion? aerodynamic? control? such and such." How ı can determine this? I think ıt will be better as quick as ı determine.

(I am sorry for my language. I guess have poor english. I am improving 😁)