I personally hate buying books to learn programmes like this.
If you are in any way inquisitive and have good basic understanding of the way programmes like this work you are best off working from tutorials.
I personally brought two biannual copies of tutorials from computer arts (one for Photoshop one for Illustrator). Once you understand the basics these are a great way of learning some more advanced skills.
They also do a great range of free online tutorials that can be found on their website.
http://www.computerarts.co.uk/home
Check it out.
I picked up my first copy of PS at your age and just taught myself by playing around with the programme.
If you need any help just ask.
Hope this helps.