Protostar- in a nutshell

The current wisdom about the formation of a star goes something like this. first, you need a lot of matter containing various elements, mostly hydrogen, along with a lot of dust that might be the result of earlier happenings in space. 

Large clouds of such matter are visible in many parts of our Galaxy and in other galaxies in the universe. these clouds roam around, sometimes with turbulent motion, rotating and interacting with other clouds and stars. 
All particles in the cloud interact with each other due to force of gravity.
Fluctuations might lead to situation where the concentration of mass is Greater at some point as compared to the neighbourhood. 
This would lead to more and more particles being drawn towards that point of concentration. the big one continuous to become bigger. As the particles fall in together, the progress accelerates.

While this is happening, we have also to remember that the cloud was originally rotating, howsoever slowly . 
the particles that come from the direction of the axis of rotation can fall straight in towards the centre while those along the equator have to content with the centrifugal force. 
Therefore, the concentration of matter does not lead to a ball but to a disc - like shape. All this might take millions of years .In later stages , the particles falling in begin to collide with each other rather frequently and their energy becomes the chaotic energy of heat. 
Does the gravitational energy is converted to heat ;concentrated near the centre of attraction . 

This heat can be sent through infrared emissions. if the cloud is large enough, the process can proceed towards star formation of the clouds .

Before it gets there you can call the child star only a protostar. the true star phase of its life begins when the interior temperature rises to several million degrees or more . 
At those temperature the kinetic energy of protons and light chemical elements are high enough for thermonuclear reactions to occur ; the energies of particles can overcome the electrostatic repulsion of nuclei. fusion of four protons to form a Helium nucleus and a couple of positrons and some neutrinos can produce more energy per mass unit than any other fuel period ,this is the energy that ultimately enables the protostar to shine, and continue to shine till the fuel inside is exhausted .
 Depending on the mass of the stars that might take a few million years to many billions of years . the heavier ones Shine brighter and die earlier . our sun will continue to give us light and heat for several billion years.

Incidentally ,let us not forget the matter around the edges of the accreting disc . it is believed that planets are coagulated in those regions.