Make the effort- In order to make new friends, you have to make the effort to introduce yourself.
Listen-Be interested in what your friend is talking about, even if you aren't.
Keep in touch- I've lost contact with many good friends over the years that I regret losing contact with. If you want to be friends and stay friends, make a hard effort to keep in touch.
Surprise them- To be a good friend, remember their birthday, a small gift or a birthday card is enough to show that you care and they always appreciate it.
These are only four steps in being a good friend, but I think they cover the basics!