This is easy. First, if you don't have the Layers window open go to the Windows menu and choose it. You don't actually need that, but it's handy. It's a small "utility" window which typically floats near the right side of your screen. And open your file, if you haven't yet. With that window open, you can click on the existing layer in the image (probably Background), and then click on the Duplicate Layer button below — or press
Shift-Ctrl-D. Then, to merge that layer back down again, go to the Image menu and pick Merge Visible Layers (
Ctrl-M). You can also use Flatten Image. Alternately, if you have more than two layers, there's Merge Down in the Layers menu, which does what it says.
Gimp has completely customizable keyboard shortcuts, by the way. Look at the bottom of the Edit menu for Keyboard Shortcuts (may be mixed in with the Preferences in some older versions of Gimp). That way, if something doesn't have a shortcut, or you don't like the default, you can fix it.