What causes soda to fizz and how can it be stopped?
The soda is supersaturated with dissolved carbon dioxide since in the sealed container the soda is under pressure using carbon dioxide. A number of things can cause the formation of nucleation sites which degas the solution by forming bubbles which float to the surface. The foam is due to the surface tension of the liquid solution. The lay understanding is that the soda is "flat" when the soda is no longer supersaturated with carbon dioxide.  

It should be noted that even in a saturated solution most of the carbon dioxide is in solution as a dissolved gas and not carbonic acid. Since soda solutions are acidic anyways, this also favors the dissolved gas as opposed to carbonic acid. 

To minimize fizzing:


Use a clean glass. (Bubbles emanating from some sort on the
glass indicate that the glass is dirty.)
Use wet ice at the freezing temperature of water (not ice from
fridge below 32 F).
Cool the soda before pouring gently onto the side of the glass.
In other words do not just dump it into the center of the glass onto the
ice. Imagine trying to pour water into the glass without creating
any bubbles.