You can come out to each person individually by simply telling them that you have something to tell them, and saying "I think I'm bisexual." Don't feel like you must come out to everyone if you are not comfortable with coming out to certain people.
You could also come out by introducing your girlfriend to your friends and family if you are dating a girl. You don't have to make a big deal out of coming out. Simply state that this is your girlfriend, and if they ask questions, explain that you are bisexual and are attracted to both boys and girls.
If you are still young, it is possible that some of your female friends will assume you like them and react badly. High school kids can be immature sometimes. If this happens, you can simply ask them if they like every single boy they know. They will say they don't. You can now explain that you are the same way. Just because you are attracted to boys and girls doesn't mean you like every boy and girl you see.