I feel that for you the only way to reform is to pull out the big guns. You need to seriously man-up. It's going to be hard but you can change. Girls like confidence. Girls like a man in control. Someone who is self-assured, albeit a little arrogant. This doesn't mean you need to walk up to a girl and act like a cockhead, but it does mean that you gotta start showing some signs of masculinity. Practice speaking to a new girl each day. Keep eye contact until they look away. If you have a pip-squeak voice, change it to at least something decent.
How do u look? You don't need to be Brad Pitt, but you need to feel like it. If you dress dorky, check out what guys around you who are getting girls are wearing. If your hair is greasy and overgrown, get a modern hair cut and start washing it. Add a little sophistication by a nice watch and wear a SMALL amount of cologne. If you’re fat, start exercising now before it's too late. If you’re a rake, start a small gym program in order to get a bit of bulk for summer. Appearance is the initial part of the attraction though. In a world of asymmetric information, it is a signaling factor. Women have an amazing ability to undermine a man's confidence, to look at him and straight away ask- is he happy with himself? Get the physical side in decent order and it won't initially help you pick up, it will just set the foundation for it.
Now you say you’re shy, obviously this is a forum and I can't go on forever. There is plenty of excellent reading (books online, download em) on this. PM if you want some. Now nobody is an expert, but I can offer a little that I know generally works (not all girls are the same!!!). Girls, unlike guys, focus a lot on the way a man exerts himself, through body language, speech, movement. Slow down everything when you speak to this girl. This doesn’t mean speak like a retard, but don't talk at one hundred miles an hour. Next be confident, if this requires you to 'psyche up' before hand, and then do it. Smile, but only on the occasion. The key factor is humour. You need to be interesting. If this girl is hot, then she will have been asked out many times before. You need to differentiate yourself somehow. Make her want you, not you begging for her to go out. Girls love unpredictability. If you’re slightly arrogant but humourous at the same time, this adds intrigue and mystique. If you attempt to play the 'nice guy' and buy her things, push her to go out to dinner, annoy her every five seconds then she will lose interest automatically. There is no chase, and she has complete control. This will see you slip into the notorious 'friends' category. Once you get here, girls view you as a counsellor, someone they can confide in. They put you on the 'long term bench' (which is only usually useful for older men seeking marriage) rather then a 'present' option and your chances of progressing to a relationship become slimmer. Why? Because you know too much about them. They become insecure, it frightens them.
In summary, if you have a good conversation with this girl you NEED to keep progressing. Make jokes; sometimes even have sarcastic ploys at her. If you do get a first date, avoid making it a job interview. Be interesting, do something fun. Don’t go out to dinner and then quiz the hell out of her or talk about what you’re doing in your study/HSC the whole time. You need to show her that you are independent and that whilst you enjoy her company, your life will go on fine without her. Girls will say they hate the cocky 'alpha male' but most of the time they are lying, especially when it comes to sexual relationships. One thing, you need to stay in control and make her become the pursuer. If she gives you her number and says call me, tell her to call you. Don't follow things up directly the next day either. If you manage to maintain her interest with a few dates, keep advancing. Many will argue, but if sexual stuff comes on the cards go for it. There's an old paradigm- If you are friends with a girl you will never sleep with her, however if you sleep with a girl they are much more likely to pursuit a relationship.
Anyway, I’m still learning. It’s an on-going process. But you need to be the MAN, not go on a forum asking girls for advice. The worse thing you can ever get from a woman is their pity. If they feel sorry for you, it is an awful situation. Id prefer they thought I was an arrogant prick then felt sorry for me. Practice now, not after the HSC. Get the foundations down now; increase your confidence BEFORE Schoolies and the 5 month break along with uni etc. If you start now, this will greatly help you as you begin to socialise more.
Good luck!