Ciao Ahmed and welcome to our community!
For transportation from Bergamo airport to Milano Centrale train station, you can take the Orio Shuttle bus10. It departs every half hour11. The cost is currently 10 euros one way. Other bus options are Terravision4 and Flixbus5.
To travel from Milano Centrale to Catania, if you are able to book online you can purchase your ticket directly from ItaliaRail5. 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!