My advice: -
Please be considerate on your part. Think about it this way, if
someone goes after your own girlfriend, and she happened to be
going out with him, how would you feel?
So, I would suggest that you befriend her instead and be steady not
to let her know you're attracted to her. Try to be cocky and funny
instead. Girls love it. And in time, if she happen to like you,
good for you.
Note: During the times when you are her friend, don't do too many
things for her, it WON'T work out for the better on your side.
Trust me.
If there're any questions or suggestions, you may PM me or post it
here. I'll be more than willing to help you.
All the Best in your pursuits! 
yours truly,
S.H.I.N.