You just need to draw a Quickshape rectangle and fill it with the colour you want as the background. This can either be placed behind the other objects on the same layer or, better, on a new layer below the other objects.
Welcome to the forums and welcome to Serif-Land. We were all new once, so don't hesitate to re-visit and ask questions. As always, if you have the ability to take snapshots of what you're doing and clip those to your posts, we may be able to get answers for you more quickly.