You actually explained how you do it
After Effects uses a layer model, just like Photoshop.
So, after you drag the movie into your After Effects Composition, just drop the PNG file into the Comp and it will also become a layer. And it will be above the layer you dragged first. Of course, you can freely move layers up and down to change the stacking order.
You may want to check the Layers and properties chapter in After Effects Help.
Thank you that helped me a lot but what I really meant was which setting to use to import the .png but I looked at the tutorial. So is the correct way to import a .png graphic to use (File > Import > File) and in the options do I leave the default import as "Footage"?
Actually, "footage" is the only way for PNG files.
File > Import > File will work perfectly. There are many other ways to do that (like dragging from the Mac OS Finder or Windows Explorer), but that will work.