It takes millions of years for molecules of Hydrogen, floating around in space, to come together in sufficient mass to eventually coalesce into a protostar (the first stage). This generally requires some event, such as collision with another nebula, the shockwave from a supernova, or the wake of a black hole, to give it the nudge to allow gravity to collapse the gas into a protostar.
That's only the beginning. Another 100,000 years to million years later, when the protostar has gained sufficient mass, it becomes a very hot ball of gas called a T Tauri star. It still doesn't have enough mass, and hence pressure, to begin internal nuclear fusion, even though it can appear as bright as a regular star.
It takes another 100 million years (depending on the mass--less time for larger stars) to collapse the T Tauri to the point where it's core can begin nuclear fusion. Nuclear fusion occurs when protons of hydrogen combine with neutrons to form helium nuclei (alpha particles). This occurs in the core of the star, where the temperature is in the millions of degrees, and the pressure is extremely high. This is the point at which one may say a star is 'born'.
Read more: http://wiki.answers.com/Q/How_are_stars_made#ixzz25EyXXYgj
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