The first thing you need to do is go into your inventory (treasure chest) and make sure the box is checked that says something about sending your harvests to your inventory. You will make more money if you store them first rather than just clicking and letting them sell right then.

You can also earn money by visiting neighbors islands and picking up objects floating in the water (such as a boot, a glass bottle, etc.) You can steal from your neighbors once a day, up to 3 items per island. Just hover your cursor over an item ready for harvest and it will tell you how many are available.

If others have already stolen from this item, you may have to move to a different item to get you max of 3 steals. Keep your own island harvested as soon as crops, trees, and animal are ready to keep your neighbors from stealing from you.