On the main toolbar at the top of photoshop, it reads from left to right:
file, edit, image, layer, select, filter, view, window, help
You'll need for this operation,
file -> new - to open a new document with a transparent background
file -> save as - to save the finished document as a tga
Select -> select all
Edit -> copy
Edit -> paste
Layer -> New layer - to create new layers
Layer -> Delete layer - to delete the downloaded image from your new document
Layer -> Merge visible - to combine your new layers into one image
You can navigate through the different layers using the Layer box in the bottom right
joe