Hi, I am new to the site, but had to join to help you out as I am working on getting my Ontario car registered in Florida.
A few things you should know, which I didn't.
If you are driving across the border you should contact that border crossing and make an appointment to have you car imported at that time. It will save you a hassle later. You should obtain a letter from the car manufacturer which states if the car is EPA and DOT compliant. This letter will cost you nothing. If it is not compliant, then I'm not sure where to go from there. You will need the Ontario Registration, the letter, and the vehicle. By "You" I mean the registered owner of the car, no one else will do. When you call to make your appointment they can tell you if there is anything else you will need. CBP will give you paperwork which quotes the 'fair market value' of your car. They get this from the Kelley Blue Book website under the Trade-In Fair value. DO NOT LOSE THIS PAPERWORK!
If you car was not manufactured in Canada, USA or
Mexico you will have to pay duty on it.
When you get to Florida you'll need to call the local tax collector's office (DMV) and find out if you have to have your VIN verified by one of their field agents. I live in the Tampa Bay area and I was required to do this.
You'll need to get Florida insurance, be prepared, it's quite a bit more expensive. We transferred our State Farm from Ottawa to here.
Then you go to a Tax Collector's office with your Ontario registration, CBP paperwork, VIN verification form, Driver's license, current odometer reading and Proof of Florida Insurance.
If you plan to get your Florida DL at the same time make sure you have your passport and any immigration documents that you have. The license will only be good for one year and it will say temporary on it.
At the tax collector's office you will be required to pay tax, big surprise I know. They will charge you a minimum of 6% on the fair market value that CBP assigns to your vehicle. You should also be aware that the county you are moving to may add an additional tax on top of that. Around Tampa Bay it is an additional 1%.
That should be it. It will probably be harder for you to get your DL than to register your car. Be aware you are also required to pay a $100 fee on top of normal registration costs because you are adding a vehicle to the roads of the state.
I hope this is helpful!