The fact that you want to be shows that you are a "nice person" inside, but as you know that isn't enough. To quote Batman Begins (and the Bible) "it's what we do that defines us" and you came here for practical advice.
Warning: This will take work.
Make a list of ways you act that you'd like to change. Then every week work on one of them to the opposite extreme. For example let's say you'd like to stop being rude to waiters who provide bad service. For a week or so, make sure to tip them the same as a really awesome server. When you're at the bank, try to be extra pleasant to the teller if you're in a bad mood. The conscious and concrete effort to change can help you change your habits and even your personality.
