Okay so I know I have dated guys before but I just don't see them the way I use to.Im gay and I know that. I have a girlfriend now and she's amazing. We have dated in the past but it didn't work out. Its different now. We both have changed and we working things out.
My question is how do I tell my dad...
He doesn't know that I have ever dated a girl or at least that I know of. My mom knows but I never talk to her. My dad says he's not a homophobe but I just don't know if he's telling the truth or not.
Its not that I'm embaressed its just that he's so akward to tell...
So how do I say it to him.

You first make the choice of do you need to tell him now ? But you either go see him or call him and just tell him.

Because you're so unsure of your father's reaction, I would suggest that you work on strengthening your relationship first. Sounds like you're going to have a few obstacles to overcome with your significant other, so building a stable relationship is the priority. Once that's established, you will be in a better position to tell your father.

I am going to assume that your parents have split, otherwise with your mother knowing as you said, likely your dad would too, if they were together.

I don't see how you can do anything other than just sit him down, and calmly tell him. If you are apprehensive about facing him alone, ask a family member, maybe a sister, or an aunt, to sit with you while you break the news.

If you are certain that his reaction is going to be complete surprise and shock, try not to overwhelm him, his reaction may not be what you expect. There is always another day to talk again when the dust settles.

If you are a high school, college, or university student, seek out guidance through the school with organizations that support the gay population in their schools. They can better advise you how to proceed, or answer questions, or offer advice from the vantage point of having done this themselves. Support will be important for you.

I wish you well.

I think Jake's idea that you include another relative or female friend is a really good idea. I did this with my father in another context where I had something important to tell him. I was worried he would just dismiss me or make a smart remark and I needed him to calmly hear me, so I made sure to do it when a mutual friend was around. I said, "I need to talk to you. This will just take a few minutes." Then I just said what I wanted to say. It took maybe 15 minutes altogether and I was so glad I did it that way. I have never regretted it. But like others have said, there's no rush. Choose your time and just say what you have to say, and then really listen to him too. Don't be defensive if he acts disappointed or confused. Just acknowledge his feelings.

What you are doing is hard.
Good luck!