Being a good boyfriend is not rocket science. If you know you are a good person, and you do good deeds for others, then you do not have much to prove. Your girlfriend will acknowledge you as a good boyfriend from the very beginning. She will know that you are generally good at heart and have positive intentions for her.
But just remember, do not bore her. There is nothing worse than a dull, unimaginative boyfriend, so avoid becoming like that. Some girls think that boyfriends are boring on purpose so they do not have to make much effort. Always try to be exciting whenever you get the chance. Then, she knows that you are trying to make the relationship work.
But being exciting does NOT mean talking too much. When you converse with your girlfriend, avoid going on and on about excruciating details. Do not start blabbering about a topic that you know she does not care about. Strike her with spontaneity some times. Whisk her away to any place that is romantic. Keep her interested in the relationship and she will appreciate you more.