I'm assuming this is for a tennis match?

The white sandals are easy. Wear white pants (like you said). Problem solved.

As for the top, don't wear a t-shirt. You can't dress up a t-shirt unless you wear a sports coat over it. Do you have a red button down? Or a blue one? Preferably red. Any solid color top will do, really.

You could also just wear all white (it is tennis, after all). A white skirt with a white top. Just make sure it's bright white and that the whites match. You could dress it up with red and/or blue accessories.

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