How to be a good girlfriend?

1. Take it slow. Don't push him to call you his girlfriend after a couple of dates. Don't start talking about marriage and children before you've even met his friends. The beginning is a precious time when you both feel like you're walking on air just because you're together. Eventually this phase will pass and you'll hit bumps in the road that will test your relationship and during those times you'll both look back to the memories from the beginning to remember why you're in it. So don't bring up any issues prematurely and make the beginning of your relationship stressful. Let it take its natural course. Cherish your time together, it's the only time this person will be new to you. That doesn't mean you should be an obsessive girlfriend and crave every second of his time. Relax. Be patient. Enjoy.
2. Be honest. While being honest to your guy is very important, to the point of being paramount, it is equally important that you are honest with yourself. If you overreact or make a mistake, you can acknowledge your error and apologize. If you're feeling vulnerable or upset, you can sort through your feelings and verbalize them to him in a non-accusing way. And the most important thing here is that you will open your feelings as much as possible. For example, if someone crushes you, tell him immediately for you two not to have problems as much as possible. 
3. Have a positive attitude. If everything you say around him is a criticism or an attack, he will not look forward to seeing you. However, you don't always have to agree with him just because he is your boyfriend. Tactfulness is a better strategy in mature relationships. No matter what, have a good sense of humor. Have inside jokes together. Be spontaneous. Be happy.
4. Make your desires, needs, and opinions known, even when they may conflict with his. You don't and shouldn't exist solely to please him. Besides, showing that you are your own person with your own needs, desires, and approach to life will keep him interested in getting to know and understand you as a person. Just remember to express yourself in way that doesn't attack anyone else's opinion or lifestyle in any way--you can be humble and outspoken at the same time.  
5. Let him live his life. If you feel entitled to all of his time and attention, learn how to not be an obsessive girlfriend. Don't be an "over protective girlfriend" let him go out without him feeling watched. Remember that he doesn't need you for everything and that you are separate people as well as a couple. When he needs some space, don't take it personally. But do make sure he knows that you are always there for him.
6. Give him gifts. When you're in a relationship it's fun to give each other small gifts. Boys love to get presents and giving him one shows that he means a lot to you. Don't overdo it though. You don't want him to think you're crazy.
7. Be yourself Don't lie to impress him, don't try to make him jealous, don't lead him on if you're not into everything. These things are wrong, and if you feel the need to do that then you don't know the meaning of a true relationship. Be honest and open with him. If the two of you are mature then you will handle it correctly.

Quoted from http://www.wikihow.com