What is the first event that occurs when a virus infects a host cell?

The first thing that virus does when it enters the host body is that it attaches to the cell membrane. When a new virion bud is produced wrapped up by the lipid bilayer of cell membrane, it contains all the proteins which were embedded in the lipid layer of the host cell.

Viral attachment with the membrane can be direct or in-direct which happens after it being swallowed into an endosome.