Flirt with her. Whisper something in her ear. Lean forward and touch her arm or hand. Use eye contact. Hold her gaze for a few seconds then slowly look away. Look into her eyes again. Don't stare because that's just creepy. Smile when she is talking. When you go out in a group of friends, let her catch you looking at her throughout the evening.
Listen to every word that she says. Ask her questions about herself. Ask about the things that she likes such as her favorite band, movie, or book. Give her sincere compliments and try to be as specific as possible. If you can make her feel good about herself, she is likely to want to spend more time with you.
Change the environment in which you hang out. Try going to a romantic restaurant instead of your usual hangout. Stay by her side when you go out as groups of friends.
Look for signs that she is beginning to like you as more than a friend. Some signs to look for include sitting or standing closer to you, lingering gazes, and reciprocating flirtatious behavior. Once you begin to see signs of interest, move on to the next steps.
Discuss what she thinks about the idea of friends becoming romantically involved. Be prepared that she may not be willing to go out on a date because she doesn't want to risk the friendship.
Tell her that you enjoy spending time with her and that you would like to get to know her better. Let her know that you are interested in getting to know her on a more personal and intimate level. Ask her out on a date if she admits that she has feelings for you.