You can jump right into Homecoming with the films you have seen. There’ll be some vague references to events outside of it but they don’t affect the plot. If, however, you do want to start with Spider-man’s first appearance. I would highly recommend watching the Captain America trilogy. Some find the first film a little too cheesy (I loved it) but it sets the foundation for Winter Soldier and Civil War which are entirely different beasts and some of the best films in the MCU.