(I'm assuming you are talking about the recent series of Marvel films staring Chris Evans).
If you just want to know about Captain America story line you can watch this series:
Captain America: The First Avenger
Marvel's The Avengers
Captain America: The Winter Soldier
Avengers: Age of Ultron
Captain America: Civil War
Avengers: Infinity War
Avengers: Endgame
Now the issue will be that there are lots and lots of characters who you won't know the context for, but it should make a reasonable amount of sense.
For the full context you would want to watch the full list of twenty something films in the
Marvel Cinematic Universe.