Pansexuality could be considered a broader category. Generally those who identify as pansexual are not attracted to an individual’s gender. It seems to be more personality oriented, or character attributes-based. It is sometimes considered a variation on bi-sexuality, since the two have so much in common.
Differentiating the two could be summarized as such: bisexuality implies a person is attracted to two genders: female and male; pansexuality implies an attraction toward both female and male, but also toward ‘anyone in between’ which includes people who don’t identify as one gender or another, transgender, transsexual, intersex, asexual and other varieties of gender where the lines may be blurred.
Pansexuality seems to be a bit less widely known. This might be part of why you do not feel like you fit in. The LGBT community is very welcoming, and also has a very diverse membership. You might want to look into it a bit more. You might find some friends, or others who feel the same way you do.
13 is a young age, and it has to be stressful to be dealing with all the feelings and emotions that are coming up. Being young though does not mean that anything you are feeling or thinking is wrong. You know yourself better than anyone else, and just need to trust in that.
Coming out is a tough process. People might, as you say, ask you how you know you are pansexual. Try to explain that you are attracted to whom you are attracted to. It has less to do with a word label than your actual feelings towards different genders.
Try to give some thought as to who you feel comfortable with, and trust. This might make it easier for you, and less stressful. That might be siblings if you have any, other family members such as parents, cousins, aunts, or uncles. If you have any good friends you may want to broach the subject. Perhaps bringing up the topic of pansexuality and seeing what their reaction is.
Does your school have a counselor that you can talk to? That might be an option to
explore if you feel comfortable with it. They might be able to give you some resources.
It is tough to feel like you have to lie to your friends and family but luckily there are several organizations, such as Alterheros, and others that are here to offer guidance and support to you.
Suzanne at Alterheros