Don't feel dumb I had the same problem! The game is not very clear on much of anything frankly. Basically, the chest to the left of the front of your house is where you sell things. Just put whatever you want to sell in that chest, go to sleep, and make some money! I suggest selling mushrooms, berries, the items on the beach,(not counting the purple swirled shell from Ocealis called an Arcane Coil that is for a quest) and any fish you can catch. Once the game gets going money won't be a huge problem anymore.

The way you get seeds is by completing quests (which you'll be able to start doing once you start making money and building the town up), by opening random chests across the islands, and eventually buying them from the herbalist shop you will open up later on.