Build fleets, many great fleets. Just like in history, Sicily's dominance relies on its control of the seas. Make sure neither the Italians nor the Byzantines are a danger, and lord it over anyone who dares to oppose that thought.

As far as conquest goes, Sicily is best suited to using your naval dominance to grab whatever good province you can if a good opportunity presents itself. From these beginnings on the mainland you can start expanding your empire. If it fails you can always retreat and fall back onto your fleet.

In conclusion: rely on your position geographically. Dominate the seas, and think from that point of view. So long as Sicily can cut the Mediterranean in two it can't lose.