We see gas clouds in many different stages of size, temperature and density, but it seems that stars form when portions of these clouds become unstable to gravitational collapse. The details are very complex and there are many different physical 'triggers' that can cause parts of clouds to begin to collapse. The cloud, globule, or core, is often only a fraction of a light year across at the start, with a density of only a few thousand atoms pre cubic centimeter. After a million years of less, the density of the center of the cloud has increased many million-fold and the collapsing material forms a central 'proto-star' mass usually surrounded by a rotating disk of residual matter supported in the proto-stars equatorial plane by the centrifugal force of rotation. The proto-stellar core continues collapsing until it becomes opaque to its own radiation ( mostly infrared) and then slows down collapsing a bit...but by this time so much mass has assembled that the core temperature is over 1 million degrees and thermonuclear fusion begins to take place. The young star then rearranges itself internally with large convective regions near the surface to transport the heat out to the surface. The whole process takes a few million years from start to finish!