Focus on fields in which you can become good at.
vs
Focus on fields that you can become good at.
Focus on fields in which you can become good at.
vs
Focus on fields that you can become good at.
The second sentence is grammatical and idiomatic as it is written. The first is not, because it has a double preposition. The first sentence becomes grammatical if you drop the final "at".
Focus on fields in which you can become good.