Deleting my whole picture instead of selection...Why?

[Ofnuts]

If [Delete] (or Edit > Clear) deletes the whole image, then either you have a full selection or no selection at all. After that, the deletion happens on the active layer/mask/channel so it's always worth having a look at the Layers list (highlighted layer) or the status bar at the bottom (but this can be masked by some tool infirmation if the pointer in on the canvas):