First, you will need 2 plugins,

1. FastAsynWorldEdit
2. WorldGuard

Then, you will have to select an area with a wooden axe (right and left click).
Once the area is selected, follow the commands:

1. //expand vert (to select the whole area up to the maximum layer)
2. /rg define (name of the region) (to create the region)

And that's it, you would have your spawn protected