I think it could be really cute, especially since youíre young. Just make sure that youíre parents are okay with it first. I have naturally black hair and I bleach sections of it and dye it purple (and sometimes magenta) with manic panic and punky colors. If youíre inexperienced its always better to go to a professional. Just make sure that the person knows what theyíre doing. Donít just go to the random hair salon on the corner and let them dye youíre hair. Do some research before you decide on a salon. Talk to the colorist; make sure he or she knows what you want. Bring pictures that are similar to how you want your hair to turn out. This is really important, especially when you want unnatural colors.
Since youíre hair is dark brown, it is likely that they will have to bleach it to get blue and purple to show up. Bleach can be damaging, especially if you donít take good care of your hair. If youíre unsure about taking the plunge, you could always try extensions. Wear them for a little while and see how you like it. You can always take them out if you decide itís not for you. You can buy pre colored hair, or you can get a blond shade and dye it yourself with manic panic. That way you can customize the color. You have a ton of options. Just do a little research and decide what is best for you. Good luck!