If you're playing the early period, it'll take a while for anyone to get trade going. It goes quicker in the high period or especially in the late period, when more ports are available at the start.

But to get trade going quickly, try getting ports and dockyards built on your islands as quickly as possible, and have them crank out ships. This way you're not tying up your land-based provinces that can be building armies for you. Also, you'll need ports on those islands if you ever want to base troops or agents there anyway. Once you have a few ships, connect Constantinople to the Adriatic, then to Genoa as quickly as you can. After that, expand to wherever there are ports, and be building ports and merchant buildings in any coastal/island provinces you own that have trade goods. Within 60 years or so you should be able to cover every coastal province on the map if you're not at war, and your trade should be huge.