To ask a girl to be your girlfriend you want to start out by expressing to her how you feel about her. This conversation should be done in person. Then you want to let her know that you are interested in dating anyone else and you would like to make your relationship exclusive.
If you are planning on asking a girl to be your girlfriend you should first ask her if she likes you. Liking is a very important part of any relationship. If the girl likes you then some flowers are a helpful tool but sincerity is more helpful.