A common toilet may use gallons of water per flush. And you pay for it all. But a modern toilet will flush on about 25% of that.
Go to a home improvement store. Look for a product like HydroRight, less than $20 USD. You will need about ten minutes to put it in. You will use no tools. You will not need to turn off the water.
First, read all the instructions before starting. When you remove the product from the packaging, there's a white cable tie on it. Don't cut this! It's not packaging. It's part of the product.
Flush to lower the water level, and then unsnap and remove the flapper. Yes, the water just keeps running continuously into the bowl. We'll stop it soon. Insert the device in place of the flapper, and attached it to the vertical fill tube, tightening it with the white cable tie. Now the water stops running out the bottom. Remove the flush handle, and run the cable from the device over to the handle hole. Insert the back-side of the new flush button in the handle hole, and snap on the front of the new flush button. Calibrate according to the instructions, until the toilet just barely flushes using the top half of the new flush button, and using the least water with the bottom half (which presses the whole button).
Using the top half of the new flush button, you'll use about 25% of the water in the tank. Using the bottom half , you'll use about 66% of the water in the tank.
Spend less money, help the environment.
