As a future Summer intern, here's what I've learned:
Look for any local fashion contests that you can compete in. Great on your resume and the fact that you are out there exposing yourself lets them know that you are confident in your designs.
I agree, calling professionals and asking if you can volunteer/intern/shadow (I recommend not in the summer, because competiton is FEIRCE), is a great idea.
Make some showstopping garments. You're 15 right? Maybe you can enroll in FIT's or Parsons Summer Programs? That would look great on your resume as well.
Retail exprience is great too.
On how to actually construct a resume, go to vault.com or monstertrak.com.
"Talk to the mirror, oh choke back tears, and keep telling yourself that, "I'm a diva!"
- Panic! At the Disco