A regulation bat, either wood or aluminum.

And make sure it's vaccinated.

A baseball. Or, you know, a really hard orange.

A glove fitted to your position and sized depending on your handedness.

Spiked baseball shoes. They're not strictly necessary, but they make you look really cool when you're sliding into base.

A ballcap to keep the sun out of your eyes. Preferably one without an obscene logo on it.