BC, if you haven't watched most of the other Marvel movies then don't bother with Endgame.
Much of the pleasure of Endgame comes from wrapping up character arcs that have been a decade in the making. If you don't know the backstory, it would be like watching the series finales of Game of Thrones/Breaking Bad/The Sopranos/etc. without having watched the shows.
I don't think the superhero genre is played out yet, at least not the Marvel movies which have more humor and humanity than the DC films.
Snorlax has a good writeup but I disagree with him on the need to see several of those films.
Knowing what I know about your general age and interests, if you just want to see if you're interested then try these two films first:
- Iron Man 1 (the movie that started it all. It's a good example of the rest of the movies though a bit violent.)
- Guardians of the Galaxy 1 (more funny and weird than the rest of the Marvel movies)
If you like them then then move on to these three:
- The Avengers
- Doctor Strange
- Avengers: Infinity War
These three are fun but not all that important to the overall story. They're OK to watch with family though they might be a little scary for younger kids:
- Spider-Man: Homecoming
- Thor: Ragnarok
- Ant-Man
If you're still interested, go back and watch these films which range from pretty good to just OK:
- Iron Man 2
- Captain America: The First Avenger
- Captain America: The Winter Soldier
- Avengers: Age of Ultron
- Captain America: Civil War
- Guardians of the Galaxy Vol. 2
- Black Panther
- Ant-Man and the Wasp
- Captain Marvel
These films are not as entertaining as the rest. There's no need to watch them unless you're seriously invested in the character arcs:
- The Incredible Hulk
- Thor
- Thor: The Dark World
- Iron Man 3
Know that most Marvel movies have extra scenes in the middle of the credits and at the end of the credits. It's fun to stick around to watch them.
Also it's not a part of this larger story arc, but as superhero movies go "Spiderman: Into the Spiderverse" is wonderful. It won the Oscar for Best Animated Feature this year.