In Donut Clicker, you start with a single donut and a few clicks. The counter slowly goes up, then you begin buying buildings that make donuts for you. Before long, the small bakery is producing far more than you could manage by clicking alone.
Click the main donut with the left mouse button to earn donuts, then spend them on buildings and upgrades. The trick is knowing when to spend and when to save.

Once the bakery gets going, you will usually have several things worth buying. A new constructor gives you more donuts right away, but saving for an upgrade can sometimes give you a bigger jump.
Golden Donuts are another reason not to spend too quickly. They can add a huge amount to your supply, making them especially useful when production starts feeling slow.
You can also collect hundreds of upgrades, unlock achievements, track your stats, and work toward prestige. The numbers keep climbing, but the basic choice stays the same: buy something now or wait for the upgrade that really helps.
It is easy to spend every donut the moment you can afford a new building. Then a useful upgrade appears, and you are back to clicking and waiting for enough donuts.
Sometimes the cheaper purchase is worth it. Other times, keeping your balance for a stronger upgrade saves time in the long run. As the bakery gets bigger, making that choice becomes a bigger part of the Donut Clicker game.