What was the earliest stage in the life of a star?
As gas collapses, it heats up. Eventually it gets hot enough to shine. This stage the star is known as a protosun, and represent the first step in a nascent star. Smaller stars might shine about a million years before nuclear ignition (fusion) begins, with is the birth of a true star.
Protostar is the earliest stage. This is when the light and heat given off is caused by the friction of the gases passing each other and not by nuclear fusion. This part of the stars life usually lasts about 150,000 years, then it condenses enough to being Nuclear Fusion and a real star is born. See related question.