Can someone give details about what is the best way to contribute? Is there some easy way like just installing an app in my phone and receive popup notifications to answer some quick questions when I'm in a queue of a random local shop?
More specialized, but perhaps more important for certain people is wheelmap.org. It catalogues amenities and their wheelchair accessibility. It has similar model for contribution as StreetComplete, you answer question about stairs, door widths and toilet accessibility.
Have fun mapping :)
Edit: corrected autocorrect.
On computer you can also open https://www.openstreetmap.org/ and navigate to are you know well and press edit (tutorial should appear).
There is also Vespucci on Android and JOSM for PC for more powerful and more complicated editing.
All editing requires registration for user account.
Feel free to use one of https://wiki.openstreetmap.org/wiki/Contact_channels if you want help from a community (and "I am confused newbie" is a good reason to ask).