Nepal's busiest flight path given takes you from Kathmandu to the mountains of the Solu Khumbu, landing in Nepal's iconic Lukla airport. Flights to Lukla depart in "phases", where one phase typically consists of four flights. Your boarding pass will designate which phase you belong to and you’ll be put onto one of the flights within the phase. Kathmandu-Lukla flights — as with any mountain flight in Nepal — depend on weather, so you'll always want to build in a day or two buffer on either end in case of flight delays.
You can also take a jeep or public bus from Kathmandu to Jiri, a town in the Lower Everest region, and start your trek from there. It often takes 3-4 days to reach Lukla, and you'll see far fewer trekkers, have most of the trail to yourself and pass through beautiful Sherpa villages along the way. Many of the guides and porters that you'll see in the Upper Everest region call this region their home.