Getting a white background behind an object

[Blighty]

Use the selection tools to make a selection of the background. The first 7 tools in the Tool Box are selection tools (see attached image). More info here:
https://www.gimp-forum.net/Thread-Gimp-s...-beginners
https://www.gimp-forum.net/Thread-Painle...tion-tools

Once you have done this the best method is:
Make sure the item layer has an alpha channel Layer > Transparency > Add Alpha Channel
Delete the background Edit > Clear or press the delete key
Add a white layer below the item layer
Save as .xcf
Export as jpg or png