I’ve never been best man for my Dad before, so I can’t help you there. But I have been a best man a few times, so maybe some generic tips will help.
Spend the wedding day, before the wedding, with your Dad. You’re his right-hand man, and he may ask you to run a few errands: pick up the tuxes, get the boutonnieres, and so on. Or he may just ask you to put on some coffee. On the day itself, before the actual wedding, whatever he wants, whatever he asks you to do (within reason), make it happen.
Make sure he looks good, and does so with enough time to get dressed and get to the church (or wherever)–my best man made sure I’d been to the barber’s and shaved and showered and so on with enough time to relax over a cold beer before getting dressed. We had only the one beer, but that was enough of an incentive to get me moving to get the necessary tasks over with, and still have a little time to relax. Might be an idea for you.
Know where the ring is at all times until it is on the bride’s finger. That’s part of your job too.
The toast isn’t difficult. Really, all you need to do is to mention how pretty the bridesmaids are, and toast the couple. Anything else is gravy. If you’re comfortable and experienced with public speaking, you can add more–humorous stories about the couple (avoid anything insulting or risque though), how they met, and so on. Being your Dad’s son, you might also mention that you’re welcoming the bride into your family. But this is all up to you. Your speech can be short or long, as long as you hit the points mentioned above.
I’m sure there are other points, but these come to mind as the most important. Hope they help!