After the harvester finishes harvesting, unload its container and the trailer. As you will probably notice, it won't be full, because there wasn't enough crop. Before setting out to sell the product, you should take some of the supplies from your warehouse (depending on the level of difficulty) and go to the store with a fully loaded trailer. Place the harvester on the nearest field, as you will need it there in the nearest future.
To use the supplies from the silo, you have to drive to it. It is located on the farm, close to the field which you've just harvested. Place the tractor in front of the silo and press the key responsible for loading [R]. With a full trailer, you can go to the store, but you should also check the current prices before you set out. You will earn the most in the Windmill or the Grain elevator. Notice that a 20$ - 25$ difference for a ton, will give you about 200$ more. At the beginning, it may seem as small amount, but when selling larger crop, the sum will be of course much bigger.
No matter of what purchase point you choose, the selling looks the same. You have to drive the tractor onto a special spot, where you can unload it. After a short while, all the grain will be moved to a container and the money will appear on your account (on normal level of difficulty, it will be about 5600$). Not bad, and it's only the beginning. Take the tractor with the trailer and return to field no. 24, when the sowing machine will probably have completed its task.
When the sowing process ends on field 24, speed up the time to the maximum value, so you won't have to wait long for the crop. Then, perform an exchange - Send the tractor with the sowing machine onto field no. 38 and get the cultivator (as soon as it completes its work) to work on field no. 29. In the meantime, you should see some crops on field no. 24, so begin to harvest it right away and also remember to slow down the time.