If you have a 3DS you can take screenshots by using the rosalina menu. You can access it by pressing L shoulder+Dpad down+Select. The screenshots should appear on the SD card.

if you wanna post a screenshot, it must be to the community the game is in. anyways u just have to press the home button when u wanna screenshot go to miiverse (dw this doesn't take you out of your game) then click on the image icon when posting to that community