Ljubljana has its own airport, which offers a good amount of direct flights. The simplest and fastest way to reach Ljubljana from the airport is the shuttle service GoOpti (www.goopti.com/en/). You can book the transport when you land (you will see them as soon as you exit the customs area), but booking in advance is typically cheaper - and is needed for the return flight in any case.
You can also fly to airports in neighbouring countries, such as any of the Venice airports (about 3:30h drive from Ljubljana), Trieste (about 2h drive) or Zagreb (about 2h drive). From there, there are bus options available that get you to Ljubljana quickly and smoothly (Flixbus), or choose the most commonly used shuttle service, GoOpti (as mentioned before), that drops you off at your door.