I hear you. I had the exact same arguments with my mum, from age 6. She'd take my brother & I clothes shopping and we'd go to the boys' section and look at all the cool stuff for my brother; then she'd drag me to the girls' section and show me dresses. I definitely didn't want them and I always asked for some of the cool stuff from the boys' section; she'd refuse and she told me I was only allowed girls' clothes.
So I started off by buying shorts & T-shirts from the girls' section in the least girly patterns available. But the more I did that, the more she pushed me to wear dresses. She sensed she was winning the argument because I'd let her have her way a little bit. So she started insisting on only allowing me really girly, flowery things instead.
So as a result, I just refused any and all girls' clothes, even if it meant she couldn't buy me anything. She'd pick up a bunch of dresses and say "We're getting these!" and I'd say "You can buy what you want, but I'm not going to wear them." And I didn't; I'd just wear my brother's hand-me-downs instead, even if they were full of holes.
Your dad is pushing against you because he doesn't think it's appropriate for 'his girl' to appear masculine in public. He thinks you're wrong in what you're doing, and he believes he's doing the responsible thing as a parent by trying to correct your behaviour. It's possible that he thinks you're just going through a phase, and that he needs to guide you back onto what he considers to be the right path for you.
So what do you do?
The advice given above is perfect. Your dad won't come round on his own; he's going to need to hear it from all sides, and he's going to need to hear it repeatedly. If you've felt this way for years, you need to let him know that. Let him know how long you've been hiding it because you were scared of bringing it up, and tell him that it's not a phase and you need to see someone about it. If you can, find a gender therapist in your area (there's a list here at Susan's) and give him their contact details. Bombard him with resources on being trans; invite him to join the 'Significant Others' board here to speak to other families of trans people; get in touch with PFLAG and ask them for advice on how to approach him; join a local LGBT youth group; speak to your school counsellor. But above all else, be consistent because if you stop talking about it or you allow him to buy girls' clothes for you, he'll presume you've outgrown your 'phase'.
Oh, and print this out for him: it explains how his denial of your gender is actually harmful to you:
The Family Acceptance ProjectGood luck
