Approaching a girl you don't know can be intimidating at first for any guy. However, if you cultivate an interest in girls as people and let go of cheesy pick-up lines and attempts to look "cool," talking to a new girl becomes as easy as talking to another buddy. If you become the kind of guy who knows how to talk easily with girls, you'll be an enviable pro when it comes to talking to women.
Observe the girl you want to talk with as a person rather than just a set of attractive physical attributes. Notice what seems to interest her. For example, if she is carrying a book, see if it is a textbook or a book she might be reading for pleasure. Other clues to her interests might include a t-shirt with the name of a band, a sports team, a university or a vacation destination. You can make a comment based on any of these things.
Notice the surroundings and circumstances. Conversations between strangers can start with observations about the weather, about a bus running late or about the unfortunate choice of background music in a store. If making a negative comment about something -- the lateness of the bus, the drizzly weather -- make sure your overall attitude remains positive and friendly.
Speak to the girl in a calm, natural, yet friendly way. Make normal eye contact as you talk with her, rather than looking at her body or turning to look at the reactions of any of your friends who may be present.
Exhibit respect for the girl by asking about her opinions and interests. Make sure you listen as often as you speak during the conversation. As much as possible, focus on making your talk an exchange of ideas and experiences rather than offering a lot of compliments on her appearance as the latter can make you sound insincere.
Let humor arise naturally from the flow of conversation rather than straining to make jokes. Forced humor makes you seem nervous. Think of the girl as a potential friend whom you want to get to know. This way if you ask her out, it will be clear you are genuinely interested in her as a person.