Star formation occurs when clouds of gas, molecular clouds, form within galaxies. Slight structures form within the clouds and some, once they are dense enough, collapse under their own weight to form disks with what is known as a protostar in the center. Dust and gas begin to be pushed away from the recently formed disk in the form of bipolar outflows- the pushing away because of energy from the clouds' falling and from recently formed stars' wind. The protostar then gathers the materials that are not being lost to the disk via the outflow, and grows. The protostar starts to undergo fusion and at that point is considered a full star, even with its continued growth. This process is referred to as gravitational collapse. There is also another method known as competitive accretion in which the stars start out small and grow by accreting material from close clouds of gas and occasionally colliding with one another.
http://www.gps.caltech.edu/~gab/astrophysics/starformation_mccaughrean.gif
http://www.physicsoftheuniverse.com/images/bigbang_star_formation.jpg
http://www.aerospaceweb.org/question/astronomy/solar-system/formation.jpg
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