to be good at sports is better :)

In the phrase "be good with sports," the word "with" is a preposition that is used to indicate a relationship or connection between the subject ("you," implied) and the object ("sports"). In this context, "with" implies that the subject has a certain level of proficiency or skill when it comes to sports. It suggests that the subject is comfortable with or has a good understanding of sports, and is able to participate or perform well in them. Another example of the use of "with" in a similar way would be: "He's good with kids," which implies that he has a natural affinity or talent for interacting with children.

to be good at doing sports