It's about "level" knowledge. As you progress through each part of the dating phase (getting the number, asking her out, taking her out, physical touching, going for the kiss, getting the next date, having sex, etc.) she's going to know that you have an interest in her. However, what she won't be aware of is how high of a level your like is for her.
This is the part a lot of new guys don't realize, and why they fail. On the one hand, they're afraid if they ask her out the girl will "know" that he has a like for her, reject him, and he won't get a chance with her. On the other hand, they end up trying to give girls these big ol' speeches about how much they like them, and end up failing that way as well. Why? Because VERBALLY TELLING A GIRL YOU LIKE HER LETS HER KNOW YOU LIKE HER 100% - thus, there is nothing for her to figure out about you in terms of your level of like.
Asking the girl out should never be a fear, though. If you ask her out, she will know you like her, but won't be aware of the level of like. She might think, "ooh, he asked me out, he must like me" - but then that thought is followed by, "but just how MUCH does he like me? Is it at 85% or 40%? Am I the only girl he's asked out this week? If he takes me on a date, how much will he try to impress me? Or, will I need to do things on this date to raise whatever interest level he currently has? I must find out!"
On your end of things, you can't put so much pressure on yourself about asking for that first date. At the end of the day, the first date is really more like a job interview. When you ask a girl out, you're basically saying: "Hey there - so, I find you attractive and think you and I could be something, but I'm not sure. However, I'd like to find out - what's say we do that through some conversation over coffee or lunch/early dinner?" If you view a first date as such, you will see that there is really nothing emotionally heavy to be worrying about. Aside from thinking she's cute, you don't really know much about her to gauge how high of an interest in her that you have, so all those "lovey dovey" feelings for her that you THINK you have shouldn't even be there in the first place - thus, taking the pressure off yourself and putting more focus on the "getting to know her better" aspect of the first date.