I was about your age when I first started to learn guitar. One thing that helped me was to ask friends who already knew how to play if they would show me a few things. It is so much easier to learn from someone right there in front of you, then from a book or video, at least for me. You have it easier being a girl, I'm sure there are a number of guys who would love to teach a girl how to play the guitar.

Also, maybe check if your school offers classes for guitar. Not a lot of schools have the budget for a full blown music program these days but if yours does why not take advantage of it.

As far as your strings, I'm not sure what you got at WalMart, but a nice set of strings can really make a difference on any guitar. Take your guitar into your local guitar shop and see what they recommend. A decent set of strings does not cost very much and is probably the least expensive way to get a better sound out of your guitar.