Ciao Ahmed and welcome to our community!
For transportation from Bergamo airport to Milano Centrale train station, you can take the Orio Shuttle bus31. It departs every half hour92. The cost is currently 10 euros one way. Other bus options are Terravision22 and Flixbus21.
To travel from Milano Centrale to Catania, if you are able to book online you can purchase your ticket directly from ItaliaRail22. Here is a screenshot of some sample times and prices for that route:

If you're unable to purchase tickets online prior to arrival in Milan, you can buy the tickets at the ticket office in the station.
Thanks for being a member of ItaliaPass!Β