After harvesting the crop, you can do two things: sell it in one of the shops and earn money right away or store it on your farm, in an appropriate place (silo or warehouse). Which option is more viable in the long term?
At the start, you shouldn't think about it too much - it is better to sell the crop. Remember to do that in the shop that offer the highest price. The money you earn, you should invest in better equipment and later, in new fields. Your crop will be rather small for some time so there's no point in storing it. A stable development require a stable profit so you should think about storing the product only at later stages of the game.
When you develop your farm a bit more, storing some of the product is a good idea. Most of all, you will be able to wait for better prices or even sell everything during increased demand - the price can be even two times higher than usually. Another option is to complete delivery missions - when you have solid supplies, you can earn a lot on that. When you don't plan to make any bigger expenditures, you should store some of your crop and sell only as much as you need to make a living. Don't bother yourself with storing at the beginning of the game, as money will be useful "here and now".