Having a teen I can understand this. It is "normal" to feel this way. Remember that you because of your age - your hormones are going crazy. You may or may not be bisexual. It could be simply "normal" hormones so don't panic and think because you are right now attracted to girls sometimes that you may be bisexual - it could be just you are a teenager.
That being said as you get older you will figure it out. Meaning you might or might not be bisexual - the reason I put "normal" in quotes is that normal can be subjective.
Right now you don't in a sense have to decide - you are learning and discovering who you are. As you develop into a mature adult you will be in a position to decide exactly how you feel. At that point I think it is better to put a label on how you feel you are - straight, bisexual, etc. Also at that point you will be an adult and better prepared to discuss your preference to your family.