If you're sure you want to take the relationship further, tell her. Remember, only do this if you're okay with the possibility of losing a friend; it can get very awkward and uncomfortable if she knows that you want a relationship and she doesn't feel the same way about you. Because what you're doing is risking a friendship to get a relationship. Some girls will be mad at you for doing this. You might not be friends after you tell her.
If you are sure, you need to tell her. Be honest with her and explain your feelings. You can say something like: "I've really valued our friendship these past couple months, and have grown to see how special of a person you are, and how alive you make me feel inside. I value our friendship over everything, but I really like you. I can't hide my feelings for you anymore. Would you go on a date with me?" Give her an out. Let her know that it's okay if she doesn't feel the same way, and that you can take it. You want her honest opinion, not her sugarcoated idea of what you want her to say. If you give her an out by saying "I know you may not feel the same way about me, and I respect that," it could go a long way to saving the friendship if she doesn't want to pursue a relationship. And if she does want a relationship, it will reinforce to her how sweet and respectful you are when it comes to her. Don't get too emotional. Whatever you do, don't break down. Emotions are powerful things, and she might think that you're using your emotions to manipulate her, causing her to distance herself from the situation. If you get emotional, she'll tend to pity you and revert back to the friend-zone that you want to try to get away from. Stay stoic, even if you're boiling up inside. Smile at her, regardless of what she says, and be as happy as possible.