Empathise with the feelings, let her dress how she wishes, have her hair however she wishes, slow everything right down other than that.
Do not focus her wanting to be a boy; do not argue about it, don't feel the need to constantly check in with her about it, even in a supportive 'I'm just being here for you' way. It's not an obsession we want to feed, even by our kindness and care.
Encourage her other interests, talk about all things not gender. Be supportive of creative self-expression in other arenas (fashion, art, music, drama etc). If she is involved in a sport or physical activity, encourage her to continue. If she isn't, find one - it's good for her to remain connected to her body, even as she goes through the process of accepting it.
Don't feel the need to take her to see anyone, unless she exhibits other behaviours indicating depression over more than 2 weeks, or self harm etc. In that case, take her to the GP (cautiously) for a referral, if needed, to an independent adolescent psychiatrist, private if possible.
Stay away from Mermaids, GIDs etc.
If you normally have a good relationship (and it sounds like you do) then continue to make the relationship a priority - spend time together doing things that are fun or otherwise meaningful.
Be the trustworthy, reassuring adult...normalise the feelings that come with puberty - it's a difficult time for many girls, of course wanting to be a boy seems like one way out of it, especially when that's the contagion of the day - but you know that she'll move through this process happy and whole, because you've been through it too, and you are not freaking out (either negatively OR positively) because there's nothing to freak out about. This too shall pass.