If you only want to learn one knot, make it the Four-in-Hand knot.
Start with the wide end of the tie on the right and the small end on the left. Begin with the small end slightly above your belly-button (will vary depending on your height and the length & thickness of your tie). Only move the active (wide) end.
Wide end over the small end to the left
Under the small end and to the right
Across the front and to the left
Up into the neck loop from underneath
Down through the loop you’ve just made in the front
Tighten the knot by pulling down on the wide end. Slide the knot up & adjust.