In addition to EB's advice:
Since you're 14, I'm guessing you're either in middle school or just starting high school. Talk to your guidance counselor about taking a home ec or fashion class as an elective. If you're not sure which class will help you, find a time to talk to the teachers that run the home ec classes and find out which classes will best fit your needs.
If your school system offers it and you qualify, you can also look into dual enrollment at your local community college, take a sewing or fashion class there and have it count towards your credits for graduation.
Check your local library and YMCA/community center as well. They may host classes there. And your local library will have a wealth of books on sewing and fashion design to give you all kinds of ideas.
Since Christmas is coming up, you can ask your parents for a sewing machine for Christmas. Nothing fancy, but a Singer Simple should be sufficient to get you started and the manual for the machine is very easy to follow.
Also, if you (or a relative) have any old Barbie or baby dolls, make some clothes for them! It's a very cost-effective way to learn and experiment!