Follow other people and signs of luggage claim, passport check etc. We had to take the elevator and go down one floor to get to the luggage claim.
Follow the signs to exit the luggage claim.
Try to find the big pink bus with the sign "TERRAVISION" on it. It usually stops in the front of the airport at the number 3 or 4.
We had bought our tickets from the airport to the city of Rome before arriving from the Terravision site. http://www.terravision.eu/airport_transfer/bus-fiumicino-airport-rome/
Bus ticket is only 4€ one-way when you buy ahead.
The bus takes you to the Rome Termini train station.