Black Widow doesn’t actually show up much in many of her fellow Avengers’ standalone movies, which makes it fairly easy to get up to speed. Here’s a list of her chronological appearances, so you can watch the movies in order and get the timeline straight:
Iron Man 2
The Avengers
Captain America: The Winter Soldier
Avengers: Age of Ultron
Captain America: Civil War
Black Widow
Avengers: Infinity War
Avengers: Endgame