There are some very important things to consider.
1 - Open the image and add an alpha channel if it does not have one. Layer > Transparency > Add alpha channel
2 - Open the Selection Editor. Select > Selection Editor. In the editor you will see as black the non selected areas and as white selected areas. You can use the selection tools over the image or directly over the editor.
3 - Very important. You should know how to use the selection tools using the "Shift" and "Ctrl" keys. Holding down the Shift key you "add" selections. With Ctrl you "subtract" selections. You can "add" selections at any time using different selection tools even with different configurations. For example, use "Fuzzy Select Tool" with low Threshold for an area of the image, and for other area use high Threshold holdong down Shift key.
4 -Then you go adding the selections you want using selection tools while you see the result in the selection editor. Remember that you can apply the selection tools on the image or over the editor. If you choose wrong areas, you can immediately revert the selection with Edit - Undo, or subtract the wrong selection by holding the Ctrl key.
5 - When you have finished selecting the background then you can now delete the selection area.
6 - Perhaps the task is easier using layer masks that have commented here, but I have no idea how to use masks.
