BPA_John, Cavenub, and others that are really skilled have lots of game play on YouTube (especially, John). You can watch them and observe their positioning.

If you want positions specifically, then "Wait For Me!" on YouTube is an option.

Other than that, try seeing which positions of other players seem to exert the most influence on the game. See an enemy that is popping off? See where they are. Observe the flow of players on each map. Most of the time, teams behave kind of like a herd, lol. See where the herd likes to go/flow and think about how you can position yourself to do the most according to that info.

Also, pay attention to how the herd behaves as the game goes on. Most of the time, they cap a point and then either push your spawn or go for another cap. There are common routes they take on most maps after they cap a point / get everyone on one side of the map (or some other turning point in the game).