Step1
Come Out to Yourself
Before you can come out to others, you must first come out to yourself and feel convinced that you are gay, lesbian, bisexual, transgender, queer, etc (hereafter referred to as GLBTQ). This is because there is a good chance you could face a lot of resistance or backlash from your parents, and you need to be able to stand firm in who you are.
The process of coming out to ourselves can take a while because of all of the [bad] things we are taughts as fundamentalists about homosexuality. It can take some time to re-wire our brains to be OK with the idea of being gay. After you have come to the point that you are OK with it, you might be ready to start telling others.
Step2
Prepare for a Backlash
The biggest thing is to remember that you need to give your parents some time to get used to it. Time could be as long as ten years or more, or less, or maybe even never. Most x-tian fundamentalists are opposed to anyone who is different or who doesn't fit into the fundamentalist box. If you prepare yourself for the worst, then anything better will just be a pleasant surprise. Whatever you do, don't expect them to jump up and down and clap.
Of note, if you are still financially dependant on your parents, it might be a good idea to wait until you are in a place where you are paying your own way in life. Some parents have been known to withdraw funding for college or just life in general.
Step3
Reading Material
For most x-tian fundamentalists, the only text they need to consider when they are researching a topic and forming an opinion is the bible. It isn't a bad idea to have some supportive reading material handy, but don't push it on them if they aren't receptive. You can let them know you have some information. But more than likely they will not want to read it.
Be prepared for a litany of bible verses and other religious references to how you are going to hell and how your life is sinful. Have some replies prepared and anticipate this. One good reply to the Leviticus argument is "Leviticus also says that shaving and eating pork are an abomination...". The key is to anticipate their arguments so that you know ahead of time how you are going to respond.
Step4
Be Yourself, Quietly
One of the best things you can do for fundamentalist parents is to live your life in a happy, successful way so you can break all of their sterotypes about being gay. Right now they think that all gays are mentally unstable, unhappy, sexually permiscuous, and demented. You don't have to do anything extravagant to quietly change the way they see gay people. All they have to do is see you doing things that everyone else does- going to work, taking care of a household (or apt, etc), being responsible for yourself, and all of the other things that make us human.
The more you try to change your parents or make them accept you, the more they might resist changing or thinking about things differently. Remember, you are challenging everything they believe about the world. Dealing with homosexuality doesn't just challenge what they believe about gayness, it challenges their whole belief system. Taking one card out of the tower can cause the whole house to collapse. This is a process of slow realizations and challenges that can be very hard for them to accept. Be patient.