So, someone asked me how I can be nice to everyone when I've been a bitch in my "past". xD So here's what I do, and what I think you should do to be nice to other people.:
1. Respect yourself.
If you don't respect yourself, you will not be able to respect others. Love yourself, your looks, your way of dressing etc.
2. Give attention.
If you give attention to people, they'll feel good about themselves. Everyone's been tried to get hurt, and knows the feeling of not being special/important. When you can give someone attention, do it. It's a great way to be nice.
When you're talking to other people, and they're feeling bad about themselves, try to put yourself in their position, what would you do? Try to understand what he/she is going through and it'll be easier for you to be nice to them.
4. Control yourself.
Had a bad day? Not feeling too well? Don't let it out on others, it'll hurt them more than you think. I've tried it myself. If you're feeling bad about yourself, or if something happened that made you really sad, tell someone else about it. Maybe they'll understand, and you'll be the one getting the attention now. - Don't overreact.
Saying Fudge this, Fudge that in every sentence will make you look bad. Try to use a language with not so much cursing, the person you're talking to will feel that you're mad at him/her. On the other side, if people are cursing at you, just walk away. There's no need to start a fight over 10 mesos, as an example.
When you're getting to know a person, ask them how their day was. How did it go at school/work, if something bad happened, ask if the person wants to talk about it. It makes you look like a listener. Listen when other people are talking to you. It isn't nice to just ignore other peoples' opinions and stories. - If you've known the person for awhile, and you know what he/she is doing, ask about other stuff such as what kinda music the person is listening to, etc.