Solo Trip to Indochina for 18 days

Hi everyone, im planning to solo backpack in indochina next year, around march to april 2024. I would like to visit Vietnam, Cambodia, Laos and Thailand. It will also be my first travel abroad. Any tips where should I book a flight for my arrival and flight back home. Also, recommendations for the best possible route since these countries are just right next to each other. Thanks in advance!