Agree with Jessica 100%! It's not uncommon for parents to be against snakes, and it's not easy to get around. The main thing is you need to put your case forward properly, without ANY cheek or sass! I say this because its easy to get frustrated and then the cause is lost. Sit down and talk to him, explain why you want one, show him all the research you've done, maybe print it out. Also, explain why you think you're capable of caring for one.
I would also suggest taking him to the pet store, breeder or even better a reptile show so that he can get a feel for what your asking for. There are also videos on youtube that are made to help convince parents, just search for them!
The main thing to remember is that if your dad is firm about not allowing a snake, then please do not make the mistake of getting one anyway and trying to hide it, or just forcing it into the house. I'm not saying you'd do this, but people have done it, and always ends badly!
Give it a shot, and let us know how it goes, good luck!!