Like he said, you can't take a small photo and some how magically create the detail in it when you scale it up. You see that in TV shows and movies, but it's complete BS.
BUT maybe your question has to do with wanting to keep it pixellated? Is that what you want? Do you have a pixel image and you want to scale it up such that the pixels remain squares instead of blurring together?
If so, the only trick I know to do that is to convert the image to INDEX mode first, then scale it up and then convert it back to RGB.