Stars begin as clouds of gas and dust, called a nebula. The force of gravity causes the cloud to collapse towards its center. As this happens, the cloud begins to rotate; this rotation creates a disk that contains a hot, condensed (and therefore very dense) object in its center, called a protostar. Friction is created as gravity continues to exert pressure on the protostar. Eventually, this friction causes the temperature of the protostar to become great enough to initiate nuclear fusion. When nuclear fusion begins, a new star is born.

The next few pages will describe what happens next in the lifecycle of a star.

Sources
http://spiff.rit.edu/classes/phys230/lectures/sf/sf.html (CC BY-NC-SA)