Lots of good information here:
https://www.aa.com/i18n/plan-travel/...tions/cuba.jsp Service to 5 cities from Miami can be booked on aa.com.
Scheduled commercial flights to Havana will start later this year. If you are going before then, try public charter carriers like
http://www.havanaair.com/ (this is the charter company that I used-- their flights are operated by Eastern Airlines 737s).