how to make a light skydome?

How do you make one?

If you’re meaning you want a dome of light then a hemilight will be able to simulate it for you.

If you want the other way, create an Icosphere, delete half of it, calculate the normals inward, make a spotlight, parent the spotlight to the Icosphere and turn on dupliverts, if you did it right the spotlight will be duplicated among all the verts.