I would like to run regular Romex up the interior wall of our garage
and then switch to conduit running along the ceiling.

What do I need to do to transition at the point where the Romex exits
the wall and enters the conduit?

I assume I need some type of brushing to hold the wire in place and
prevent it from scraping. Beyond that is it sufficient just to push
the conduit entrance up tight to the exit hole from the wall?

What specifically do I need to use here?

Thanks