Hi
That's not too difficult
Create a png circle of the colour required and with a transparent background.
Alternatively use Bing to find suitable png circle images..there are loads you can download.
Place your main clip on the sequence line Video track 1 and the circle image onto the overlay track above it,Video track 2.
Drag the red cursor line so the sequence preview shows the circle on top of the video.
Select the circle on the overlay track and click effects.
Scale the circle to the size required.
Add the Position effect and drag the circle to the required place and Apply the effects.
You will now have outlined your subject

Use keyframes to move the circle around to follow the subject if required. This works best if the subject moves in straight lines.
Nat