Photoshop - Transparent backgrounds replaced by white?

I've literally just started using Photoshop tonight because I want to edit my mum into a photo of Michael Buble for a photocake. All was going well, I managed to cut out my mum and layer her over another photo, but I wanted to make another. This time however, instead of having the transparent grey boxed background (meaning I can layer her onto another picture and still see the background of the picture), I have a white background. I don't know what's changed, but I need to get it back to the transparent one as I don't want my mum in a white box and then pasted over a picture of Michael Buble, I could do that in Word. Thanks.