I suggest to watch online tutorial videos. Check Adobe TV for a first start.
About your question: Open both images in PS, then switch to the one you want to add, press Ctrl+A (selects everything), press Ctrl+C (copy everything in the layer), assuming your image only has one layer, then this is OK. Then switch to the other image and press Ctrl+V to paste. PS will create a new layer and paste the image 1:1.
Alternatively, open only the target image and place the other image per drag 'n drop in the target image, which will let PS make it a smart object (not 1:1, but can be resized).
Then do whatever you want with the layer of the added image.