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.
As long as it’s not 60’s Batman type cheese I can handle it. Also, seeing something so different than what I’m used to can give me a new perspective on Superhero flicks and I’m going to watch it for that reason alone. The only thing that sucks is that I’m running through Breaking Bad again and that’s taking up most of my time outside of work so this is someting I’m going to have to push aside for the time being.