In addition to what Jamie has already mentioned.
The auto-focusing system will try to focus on what is central in the picture or what is closest to the front of the lens. If this isn't what you want, there is a small switch on the side of the lens that you can switch from AF to M (auto-focus to manual) and then use the focus ring to focus where you want. Another method is to put what you want in focus into the middle of the viewfinder, half press the shutter button, let the camera auto-focus, then recompose the shot (which might be fine if it's a quick grab shot where you might miss the action, but isn't recommended as you'll likely be increasing focal length between your camera and the object you are taking a picture of and therefore your focus will be out).
Alternatively, if you change the camera mode out of one of the creative modes or full auto (the green square) onto P, TV, AV or M, you can then manually choose one of the 9 focus points (press the button with what looks like a cross above it - on my Canon 500d it's the same button as the zoom, on the top right of the camera back), you will then see the little red focus light when you look through the view finder, you can move this around the focus points with the scroll wheel, until the red light is over where you want to focus. This is replicated on the rear screen (if you have the display set to show camera settings).
I suspect what has happened with your photo is that the camera has reduced the aperture due to insufficient light, which has given you a shallow the depth of field, which is leaving the dog in focus and the persons face out of focus.
Personally, I'd suggest you play around and get a bit more familiar with your camera and with your kit lens (which is pretty good and gives you a bit more flexibility and a bit of zoom), before getting the 50mm 1.8 (which is a lovely lens and one that I used to take my entry in the weekend competition
http://www.digitalcameraworld.com/fo...ead.php?t=9921 GREED)
I hope this helps.
Regards
Nick