The first drinkable glass of soda was made by Joseph Priestley, a minister, philosopher and friend of Benjamin Franklin. In 1767, Priestley lived in Leeds, England, right next door to a brewery - which you might think sounds wonderfully convenient, but actually means that most things in your house end up smelling of yeast and malt - although, if you've ever been to Leeds, you'll know your clothes smelling of yeast pretty much makes you posh.
Actually, in Leeds, clothes pretty much make you posh.
Priestley was given permission to perform scientific experiments above the vats in the brewery - a flagrant violation of modern health codes, but apparently not frowned upon at the time. He found that when he held a bowl of water above the brewery gases (which he called "fixed air"), they infused the water, making it bubbly.
Priestley excitedly shared his new beverage with his friends, but he didn't realize (or didn't care about) its money-making potential. The creation of soft drinks would have to wait for future (and more "financially motivated") innovators.