First of all you need to plow fields. To do this use the tool as shown above. If crops have wasted you also use this tool to plow the wasted fields.
Once you have done this you need to plant seeds.
Click on the Store icon and then click on the "Seeds" tab. Any seed that is showing unlocked is one you can plant. Hover your mouse over the box containing the seed you want and "Buy" will appear. Click on "Buy" then click on a plowed field to plant the seed.
This is an example of what you could see:
Top left of that window will show you how many coins you have available to spend. Don't spend them all as you will need coins to plow the fields again, plowing always cost coins.
If you have a purchased a seeder ( 1x2 , 2x2 or 3x3 ) you can plant more than one field at a time, however these tools use Fuel. How to get Fuel is explained in
Chapter 20 Fuel. Click on the Seeder to pick it up and the Seed section of the store will open. Then click Buy on the seed you want to plant and then click on the plowed fields.
As you plant each field, coins will be deducted from your coin total.
You can find out more about Crops and how to sort the seed list in
Chapter 26 Seeds.