Now is the time to develop industrialization, and military technologies. You should also begin planting cities on other islands, and do some serious exploring if you have not already done so. You need to find out where your enemies are. Go for techs good for ships, and try to build Magellan's Expedition. You want to get as much production as possible out of each city.
Pollution becomes a problem.
Once you've got all your cities going strong, you must build military units. Pick a relatively weak enemy, and send over a few ship-loads of troops. Take over his cities, and use them to build more units to take out the rest of them with. Repeat as often as necessary.