How should I connect conduit in the wall to conduit on the ceiling?
If I'm understanding your diagram correctly, I'm assuming you want to run the conduit on the bottom surface of the joist or finished ceiling, rather than inside the finished ceiling (or on the side of the joist).

Either way, one option is to make your 90 degree turn with an elbow or L below the top plate of the wall, and then use an offset fitting to bring the conduit up to the surface of the ceiling (or joist)