The first thing to consider is your desired end result. What do you hope to gain from this experience? Freedom, Acceptance, Support - these are all common answers. Before you come out you should always know what you want to achieve. If you want to come out to.
The decision to come out is yours. Make sure you are not under any pressure to come out.
Consider your age. Many people see teenagers and young adults as simply being "confused." Even if you know with every fiber of your being you are gay/bisexual/asexual/etc., keep this factor in the back of your mind and give it time. Eventually the ones you love will realize that this is not a phase and accept you for who you are.
Make sure you are educated and ready for questions. Questions are almost always inevitable in these situations. Many people, especially those unfamiliar with the LGBTQIA, will have questions. Rehearse with a counselor and/or a friend you trust before coming out to a group of people. Be as comfortable with your sexuality as possible before you come out.
Also, consider the age of the person you want to come out to, especially if they are older and do not accept people of the LGBTQIA. If you believe that coming out may jeopardize your relationship with his person, you might want to rethink coming out to them.
Always have a back up plan. If you want to come out, accept the worst. Make sure you have other living arrangements set up in case your parents react poorly and you need a temporary or permanent safe haven. Also, make sure you have other schools lined up if you go to a school that isn't welcoming to LGBTQIA students.
These are some things you may want to consider before taking this huge step. Always remember that you don't need to come out to everyone in your life; sometimes coming out to one or two is enough (if you do choose to come out).