The best way to help your daughter is to just be supportive. If it ends up she's not bi, and was just still learning about herself, there's nothing wrong with that and she got to have a wonderful parent along the way. If she does continue feeling as though she's bi, then all the better for believing her and letting her work through her feelings in a positive environment. I know it can be hard to learn your child might be lgbtq+, especially at a young age, but just know that she is not hurting anyone, and having a supportive environment can help her grow and live happily, no matter her sexuality. Maybe other, just general things you can do is to remind her that she doesn't have to force herself to understand her feelings, that she can take things at her own pace, and also about how to be safe in a relationship (no matter who the gender of her partner).
The best way that you can help her is to be supportive. She has just done a tremendous thing coming out to you. That means that she trusts you enough to know a very personal part of her. This is wonderful! I know you may have some doubts because of her age, but just be patient and understanding with her. She is going through a confusing time trying to affirm her identity, and you can help support her by being loving, and nurturing, and most importantly a steady person through all of this.
At this age, your daughter is just trying to figure out who she is. This is not a bad thing, and most pre-teens and teenagers go through a stage where they may question their sexuality. In this situation, the best thing you can do is to be supportive of her! If it turns out that she is a lesbian, pansexual, or chooses to identify as something else, that's okay too. For some people, it can take awhile to figure out who they really are and how they identify. For your daughter, that may be the case as well. But if she is happy identifying as bisexual, then it is completely okay! Being LGBTQ+ is not a bad thing at all, in fact, in is a wonderful thing! She is a part of a wonderful community, that I myself, and many others are also a part of. Don't push your daughter to figure out who she is or try to "fix" her, because doing both of those things will just hurt her. It seems as though your daughter trusts you a lot, as she chose to share the fact that she may be bisexual with you. Most people find that it is hard to come out to their parents, as one or both may not be supportive, or they fear that their parents will have a bad reaction. The best thing you can do for your daughter is to be supportive of her decision and trust her, knowing that she is happy being herself and that it is okay to be something other than straight.
Try to support her. Remind her that you will love her unconditionally. Sexuality can be very confusing, especially when you are young. Allow her to figure out her sexuality on her own, while guiding her to loving herself as well as others.
You can make sure she knows you and those who love her will be by her side no matter what, so whatever she will identify with, she's always gonna be supported and loved. And in case she has doubts, you can reassure her that any orientation is perfectly ok, and that what truly matters is her happiness! In case she's wondering how she can know for sure that she's bi, it all depends on what her doubt is. I'm bisexual, and for me the doubt was that I wasn't sure whether what I felt for women was admiration (for their body and personality) or something more. Once she identifies what is the doubt that is holding her back, she can work on it, and see if the way she looks and thinks about boys and girl is the same. Also, remind her that there's no pressure for her to find out, everyone does that at their own time, it's ok to give herself the time she needs!
Maintaining a positive and healthy relationship with her can help ensure she continues to grow, and explore how she feels. It can be difficult to grow up, being apart of the LGBTQ community. From my own experiences, the world seems more accepting, but there can be many personal feelings and experiences that may challenge her. There will be many things she will have to explore on her own, but having a non-judgemental, open and caring environment for her at home - and supporting her to pursue social activities, where she can have support from her peers, is critical.