If you have not found the tutorial, go to settings, then uploads. You will have a button there that will start the tutorial.
The onboarding now is just a series of information for you to look through. If you saw some words and pictures, that was the tutorial. If you have looked at that, restart your game, and then you should be able to make a pokestop contribution.
Let us know if you are all set after that and post a screenshot of what you are seeing if you are still having trouble.
If you are level 37 or higher, and got the prompt to complete the “tutorial,” it consists of a few pages of information they call onboarding. If you viewed these pages, restart your game. Then go back to the section under uploads and you should be able to make a “Pokestop Contribution.”