Chances are, it won't be nearly as bad as you think it will be-that's how it was for me, that's how it's been for every trans* friend I have. I think the key is just to remind yourself that nothing is going to be perfect, but to keep your head held high nonetheless. Not to be a downer, but unless you're a wizard with a makeup brush, you aren't going to pass your first time. It won't matter, anyways-you'll be scared, but thrilled, happy, and at peace too. You do need to prepare yourself for the occasional sideways glance-but besides that, you'll be fine!
Besides, your going to an Against Me! concert-everyone there will either be queer, accepting, or too drunk/stoned to care! Best of luck to you, hon! Keep your chin up, and enjoy the plunge!