I'm a slim guy and have zero fat around my stomach. what exercise can i do to work out my abdominals?

Resistance exercises: Resistance exercises are recommended.

Many: Crunches, sit-ups, leg lifts, bicycle kicks, and many others. There are good ab workouts on youtube and available by cell phone apps to help. A gym will have machines to help also, as well as incline boards to give you more resistance for your exercises. Good luck.

Add some weight: If you're trying to build more muscle I would treat your abdominal muscles like any other muscle on your body. Instead of doing a lot of situps, try doing less but adding some weight resistance. Any abdominal exercise will do but focus on some that hit upper, lower and obliques. Core exercises like planks will help as well. Increase your protein intake which will help with muscle growth.