Hi Hailey
I fully understand where you are coming from...
Its hard totally confronting and going against your parents, as they are your family and blood ties, and especially in your situation financially. Also your father is probably coming from the position of "well if you change your body and you suddenly decide to not go through with your transition then its hard to revert" but honestly, its very unlikely that would happen, as you know in your soul who you really are.
However speaking from the heart, by being constrained it would more likely be you that suffers most... As stated by Rejennyrated you are 18 and as such have control on your life and health, therefore you can go and just do it, excepting financials.
I think Jerri's comment is also a good idea, invite your father to a therapist session with you, at least he can start to understand what you are feeling and hurting about. Another thought, what about building an income stream for yourself?
I have had a long and tiring journey to get to today, not quite there yet, but for the first time in my long life i am happier now than ever before, even knowing the fall out from years of doing the right thing for the sake of others is still to come, being transgendered never goes away.
I hope you can find a way to resolve the situation... best wishes to you

Hugs Katy