I would mainly say to wear what you are comfortable in. if you want to expose your belly go for it, but it is also not at all necessary. It is helpful to wear form-fitting clothes (for example, yoga pants and a tight tank top or teeshirt). That way you and your teacher can easily see the movements of the muscles in your hips, stomach, and chest. Also, I recommend investing in a nice hip scarf.
As far as your belly being an advantage, it can be. The way that a fat jiggles during certain movements can be really beautiful. Still the belly dance world still has a preference for thinness. Find a teacher who is really supportive and inclusive for people of all sizes. If you feel your teacher is not supportive, find another. On a side note, try to find a teacher who is well-educated and respectful of the history of belly dance and the cultures it came from.
Most importantly, have fun. Enjoy the feeling of your movements. Enjoy looking in the mirror at your body as you dance.