Well my opinion is that you need a lot of support from your parents/family and that you should tell them, because they will find out most probably when you are filling your forms in and going to interviews.
Also it is better to start when your young, on stage is a good start or performing in some kind of form, but there is also one thing you need to remember, never give up your dream it only comes around once!
Hope this helps a bit.

 you should tell your parents that you really really want to do this and if they say after high school well it won't be that long.

 Well I really hate to sound like your parents but you should wait until you are older. At 13 there are to many things that you will be exploring and learning.
If after high school you still have the same passion go to college for it.
It is always best to have an education no matter what you want to do with you life.
If acting should not work out you would have an education to fall back on instead of being a waitress trying to make it into acting. You would have a college education to work and pay for you acting lessons etc.
Acting lessons and etc. Are expensive even if you have an education in it. At your age, and yes you probably have heard this before but there might be something that you find that you have the same passion for.
As you get older you can look to see if your community has local plays and a community play house to start now.
Do not throw you HS nor you college education away.

 Hello, I'm the same age as you.
I'm also pretty much the same. I love expressing myself.
 
You'd need support from friends and family, so you might as well tell them. At least you'd be doing something you'd enjoy in the future. If it's really your passion, why let something like that get in the way?
Sometimes you have to trust your heart, not another voice.
 
Anyway, good luck :)x