how can u fill a mesh like i got a shell of a house now what do i need to do to fill a face in where the roof is?
select verts, press f
In edit mode select the vertice you want to become a face (three for a tri and four for a quad) and press ‘f’ to fill.
EDIT: You beat me to it. 
yes… and if you have more than 4 verts
and there are edges between them otherwise [like the edges of other faces, or created by selecting 2 verts and pressing F]
you use shift+f to fill the region
[sometimes alt+f afterwards to make the filling look better]