Kinda a tough question Novaz...........
I tried to get a bike when I was 18 and my parents said !NO! And they told me if I did it without them then I would have to move out.
Lets just say I'm 20 now, with my own apartment and a bike

.
When you say you have no credit do you mean you'd be willing to sign a loan for one? I had a cell phone bill in my name(less than a year at that point in time) and a Lowe's store card with the worst credit and limit imaginable. I applied AT the store for a brand new Ninja EX500 last summer, and got aproved for 5,000 dollars thorugh Kawasaki financing. I don't know what you have registered in your name (I hear that cars regestered in your name are good credit too) if anytihng at all but there is a possibility you could get a loan.
If this is not anything you were willing to do, talk to your Dad a little more in a serious respectful manner.
-If you want a smaller bike like a 500, let him you don't want a 600 to get going too fast before you've gotten experience on how to handle a bike.
-Maybe you COULD get a 600, but make it a Katana, SV650 or something with 600+cc's but not straight out power.
-This is a bike that YOU want. If he's willing to buy a bike for YOU, why can't he buy what YOU want? Tell him you are being as rational as possible when looking. (P.S. sometimes with smaller cc bikes you could get a better bang for your buck with new ones other than a used one).
My Ninja I had for a year. I sold it for 600 less than I bought if for, and it needed:
-A new Tire
-New Brakes(maybe)
-Full scheduled maintenance
(NOTICE: I kept my bike pristine, but many other people may not break in a bike properly, not change the fluids or lie about accidents).