The answer depends on how you are as a person. Generally, you want to work on imbibing two qualities: 1 being able to be at ease with yourself, 2 willingness to be straightforward about how you feel towards a girl. They are always looking to get somewhere, or do something, or get something. If you have this attitude when you are talking to an attractive girl, she will most definitely get that you are trying to get somewhere with her!
So, how do you make yourself into someone who is at ease with himself? The right kind will change the quality of your life — it will make you less tense and unsettled. Getting into the groove of it is difficult though, because it takes enormous discipline, especially at the beginning. But it will be worth it. The other thing you can start doing to cultivate a sense of ease is to start expressing your emotions without judging yourself, or fearing that others will judge you.
If you are angry at someone, express it. Feeling sad? Express it. Want something? Tell it to people. We start to believe we must hold back our anger, contain our misery and hide our happiness. When you have stoppers in all your emotional orifices, how are you supposed to be at ease? Why do people become uneasy with themselves? Because they pay too much attention to themselves! They think the whole world is watching them, commenting on them, and judging them.
So all of their thoughts become centred on themselves, and they become unable to connect with others. Most people are like that to a certain degree, so if you can make yourself less of a focus in your own thoughts, you become more receptive to others, and that is attractive if it is genuine. JFK is said to be a very charming man — people often cite that when he is talking to them, they feel like they are the centre of the Universe; and if you look at how JFK carries himself, there is a certain ease and comfort with his own skin.
If you genuinely find her attractive, tell her, then ask her out. A lot of what decides if a girl likes you or not is in the impression they have of you, which is determined by the way you are, and the way you carry yourself. There's an old local joke over here, saying that you need the minimum of 5Cs to be able to snag a wife, much less a girlfriend. What are the 5Cs you ask? It's just a joke but I think it's quite true. Holds true for many girls around the world too.
