Hi Carole! Ok...I use PS7 so I hope I can help.
Let's say you want to work with a png image. This is what I would do. Open the png image and open a new document (say 8.5x11 if that's the size you scrap in). Now, select all (ctrl+a) of the png. Copy that (ctrl+c) and then paste it into your new doc (ctrl+v).
On the new doc you should now have two layers: the background and the png image. On the layers palatte, the PNG should layer should be above the background. Remember...the layers work from the bottom up. So, if the png layer is below the background you may not see it on the work screen.
Select the png layer by clicking on it once on the layers palatte. Now, try to "free transform" it. I use the hot key code of ctrl+t. A "bounding box" should appear around the png...it looks like think lines with tiny boxes in the corners at on the sides. You have a few options here...you can click on a corner and drag it out and you'll be resizing/warping your object. If you hold "shift" down while you click and drag on a corner the size will maintain proportion while resizing. Once your object is the size you want, hit "return" and the bounding box will disappear and your png will be resized.
I'm not sure why you couldn't resize earlier. Objects drawn directly on the background cannot typically be moved/altered easily. When you open a PNG, sometimes it will open as a single layer. If that happens, that means your png IS the background layer which means it's mucho tough to manipulate it. That's why I copy and paste into a new doc.
Hrmm...I think I'll stop now. I hope this helps. If not, just post back and we'll work through it. Trust me, once you get going it'll get easier and easier.
Good luck!