Well, each genre has its own unique challenges:
-For fantasy you need to make the world vividly come alive in the reader's mind, get them caught up in the mysticism and everything
-For a thriller you have to very accurately manipulate the reader's emotions (mainly anxiety, suspense etc.)
-For sci-fi you need to be able to think abstractly, and have enormous volumes of real scientific knowledge. For me, the most satisfying moments in reading sci-fi are where character A comes up with an improvised solution to a problem and is about to implement it when character B says "hey wait! You have to take X into account!" Where X is an actual scientific principle that would affect character A's actions if this were happening in real life, and upon reading this I think, "hey, you're right, character B! I never thought of that!"
The first two require you to put yourself in the reader's shoes, to anticipate exactly how someone would respond to the story if they didn't have the knowledge that you have. The third requires a lot of factual knowledge and the ability to think outside the box. The only advice I can give is to play to your strengths. Good luck!