you need a 94+ rear differential carrier, a driveshaft, and axles.
What turbored said. For future reference this info is available at Miata.net in the Garage section under Drivetrain. The Miata.net Garage section and the Miata.net FAQ and the search function both here and at Miata.net should always be your first stop when you have a technical question like this one that gets asked often. If it isn't answered there or you need clarification go ahead and ask away. Also don't forget to check out Randy Stocker's SOLOMIATA website:how can i make this work on my car?
You'll get a million different answers from a million different people. But the simple answer is for best performance get a clutch type LSD with the larger ring gear for the 1.8L cars along with the bigger housing and matching axles and driveshaft. Buy new MS diff mounts while you're at it.which one is good for daily,weekend autocross, maybe a bit drift also