You begin with a call to your homeowner/renter's insurance company. Typically, lightening strikes are covered.
As for repair, lightening strikes is the ultimate stress to a device. I don't repair lightening strikes. I don't even save the PCB boards of lightening strikes for spare parts, since I have no idea what kind of stress that component has taken. Even if it works at first, how long and will it work predictably.
If your insurance won't cover, and you want to attempt a repair, you will need to open up the TV and systematically diagnose the TV. Start with the power supply, and move forward. You will want an oscilloscope if you don't already have one, since you'll need to know the quality of power coming out of the supply. Also, be ready for a lot of soldering.
Here is a link to purchase the service manual for your TV: https://www.amazon.com/Samsung-HL56A650C...