You can either chose the multi-tool and click on any piece of plowed land which will then pop up the market where you will need to select the type of seed you want to plant. Then it will plant that type of seed on that open plowed area. Once you do this you can then keep clicking on plowed areas and planting the same type of seed.

You can also start planting by clicking on the marketplace and then selecting a type of seeds. Then clicking on any open piece of plowed land.

It's very important to choose which type of seed you want to plant by your ability to get back in time to harvest it. Just because you can make the most money early on by doing strawberries doesn't mean you will. Strawberries have to be picked every 4 hours. If you fail to get back to them before they go bad you'll be out the money for plowing and the seeds.