I think a good "healthy" friendship is marked by two qualities: balance and unselfishness.
Balance: Balance means that there is a mutual respect for each others' time and attention. You don't do everything together but have other areas of interest in your life. You can lean on each other for support but don't rely on each other to exist.
Unselfish Good friends are those who care for each other and attend to the needs of the other. They reach out to each other, are not jealous or possessive, and consider each others' needs. A good friend is honest and speaks the truth in love. This means that he or she tells you something for your benefit, even when it isn't easy to hear, and says it in love. A good friend doesn't gossip behind your back and is not superficial, but instead is real with you (does not put on a "show"). A good friend is faithful.
An unhealthy friendship is one where there is an imbalance in effort. A bad friend, for example, is selfish and self-centered and only looks out for his own interests. He or she is dishonest, and brings the other down instead of leading him or her to do things that are good and edifying. A bad friend doesn't think about the other person's welfare or consequences.
The best way to ensure that you are in healthy friendship is to first be a good friend yourself. Proverbs 18:24 "A man who has friends must himself be friendly, but there is a friend who sticks closer then a brother." (Jesus).
P.S. Don't be pressured into saying or doing anything you're not comfortable with. Self-disclosure is part of a good friendship, but a friend needs to be sensitive toward you and respect your comfort level.