Hello Anon_170859 and welcome!
I don't think noticing someone is pretty means you have a crush on them--I think it means it's something you've noticed. For instance, you may have noticed an actor or actress is good looking, or perhaps even the person bagging the groceries at the supermarket. Just because you've noticed something physical doesn't mean you have a crush.
You may have noticed other things about this girl and your other friends in the past, such as they're smart, funny, goofy, or whatever. That is part of getting to know someone.
Now, this doesn't mean that you might not develop a crush or her or someone else. I do have a resource for you that may help you figure it all out.
Is This Love? is an article meant specifically for teenage girls to help them sort things out. Even though they talk about guys, you can figure a lot of what they say applies to both genders--no matter who you care about.
You may want to talk about this with a trusted adult, or see if your school or community has a GLBT Center or teen outreach program. The Amercian Academy of Pediatrics in this article on
Gay and Lesbian Teens says "During early and middle adolescence, a youngster's sexuality may be in a state of flux."
So give yourself time and get the support of trusted adults to figure things out, and don't be in a rush to apply labels to yourself.
Let us know how you're doing,
Byroney