I will arrive at Miami airport tomorrow and have to go to Fort Lauderdale. I know I can go with TriRail, but it's in the airport?

You have to take an airport shuttle to Tri-Rail station, call Tri-Rail they would be best support Super Shuttle is another option from Ft Lauderdale to Miami, best option for one passenger

You can get help from airport staff. They will get you to trirail

There will be an elevator inside the airport that leads to the tri rail. You can ask the employees there to direct you