Quick Links
The point of Stardew Valley is to run a farm, forage the nearby woods and mountains, and in general make money by being completely self-sufficient. So while you can buy 
Basic Fertilizer , 
Wheat Flour , and 
Sugar from nearby shops like Pierre's and JojaMart, you can also make them on your farm if you grow the right crops and collect the raw materials you need.
However, making sugar on the farm may be tougher to figure out for some players since you can't grow sugarcane in Stardew Valley and you never get a dedicated sugar-making machine. But there is a way to make sugar on your farm, and it's based on a real-world method.
New Stardew Valley Player Learns About Shipping Bin the Hard Way
A brand-new Stardew Valley player learns the hard way exactly how the farm's shipping bin works in the game.
How to Find and Grow Beets
Once you fix the bus stop, either by buying all the Vault Bundles in the Community Center or by paying 40,000g to the Joja Community Development Form, you can visit the Calico Desert. The shop there, Oasis, sells a set of seeds you won't find anywhere else in Stardew Valley, except randomly in the Traveling Cart.
Beets are the fall crop on the Oasis list. Each seed costs 20g, and 
Beet Seeds take 6 days to grow. You can eat the beets, use them to craft the Vegetable Medley, or gift them to villagers (Evelyn loves them). One more thing you can do is feed them into a mill to create sugar.
How to Get a Mill
The mill is a farm building you can commission at Robin's place. It first appeared in the 1.1 update, so it's been around for a while. The cost is very low, at least after you get access to wool and looms: 2,500g, 50 stone, 150 wood, and 4 cloth.
To use a mill, all you need to do is throw millable crops into the big opening in the center of the building, and the next morning you can retrieve the milled products from the bin on the right. You can even mill several different types of crops during the same day. The three millable crops are wheat, rice, and beets. Beets turn into sugar, although the beets you grow in Stardew Valley aren't the same variety people in real life use to create sugar.
Specifically, one beet in the mill becomes three bags of sugar. Sugar sells for 50g per bag, while beets sell from between 100g to 200g depending on their quality. If you want to make the biggest profit on beets, you should sell any iridium-quality beets you grow instead of milling them. You can also turn beets into pickled beets, which have a base price of 250g no matter what quality you start with.
On the other hand, sugar is an essential ingredient in 18 recipes. If you're trying to craft every food recipe in the game, 20g for three bags of sugar is much cheaper than the 100g for each bag you'll need to pay at Pierre's. Ultimately, you should convert some beets into sugar to give yourself a nice stockpile for home cooking, but pickling them in a preserves jar is the best way to make a profit.
