Newbie Alert. Looking for a few lines of advice regarding Rotobrush two.

Trim your clip and create a comp from the trimmed clip. You just want the frames that are going to be in the final edit. You will want a separate comp for each shot.

Duplicate the footage layer, solo the top layer, and draw a mask around the top of the actor's head that just covers the background you want to remove, then pre-compose the top copy moving all attributes to the new comp;

Change to the Paint workspace, right-click on the Nested comp then select Open Layer, then apply Rotobrush. Let it propagate, then use the refine edge tool and let that propagate. Fine-tune the roto with the Effects Control Panel. As a final step Freeze the Rotobrush.

Return to the default workspace. Add a new black solid to the comp sampling the black background color. Use the nested comp (your pre-comp) as an Alpha Inverted track matte for the black layer. Turn on the bottom layer (original trimmed footage), then use the pen tool to create a mask on the black solid that just includes the background and you end up with something like this:

I hope you followed that. 

Masking and pre-composing cuts down the amount of work that Rotobrush needs to do. The Track Matte pokes a hole in the black layer to reveal the top part of the actor's head. 

If you are lost on any of the steps consult the User Guide. You always want to trim, mask, maybe color correct to improve edge detail and contrast between the subject and the background, then Pre-compose before using Rotobrush. Rotobrush should be the only effect applied to the Pre-comp. If the Rotobrush takes a long time, Pre-compose the Pre-comp again and move Rotobrush to the new nested comp, open the original Rotobrush comp and choose Composition/Pre-render to render the Rotobrushed comp and replace the nested comp with a High Quality With Alpha movie. Rotobrush is a resource hog and long pre-composed shots (or shots) that have Rotobrush applied can cause render problems and bloat your AEP project file size. If you spend more than 10 or 15 minutes running Rotobrush it is always a good idea to Pre-compose again, then render and replace the Rotobrush comp in the project. When the render is done you can delete the Rotobrush comp you rendered.