Here's how it works:

1. Giant Molecular Clouds: These are vast, cold, and dense clouds of gas and dust in interstellar space. They are the birthplace of stars and planetary systems.

2. Triggering Collapse: Something needs to trigger the collapse of these clouds. This could be:

* Supernova Shock Waves: Explosions of dying stars send shock waves through space, compressing the cloud.

* Near-Collisions: Close encounters between clouds can cause gravitational disturbances.

* Galactic Tidal Forces: The gravitational pull of the galaxy can also compress the cloud.

3. Gravitational Collapse: Once the cloud is compressed, its gravity starts to pull itself inward. This creates a dense core within the cloud.

4. Rotation: As the cloud collapses, it also begins to spin faster, due to the conservation of angular momentum. This spinning creates a disk-shaped structure known as the protoplanetary disk.

So, the first step in building a solar nebula is the collapse of a giant molecular cloud, which then sets in motion the processes that lead to the formation of stars and planets.