I would like to help newbies on a Stack Exchange site, by posting a link to a tutorial.
Can I do such a thing, and if so where can I put the link? Can I start a discussion for this, or something similar?
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Each Stack Exchange site got something known as Community Promotion Ads.
Those are advertisements that are published by the users themselves, in the per-site meta.
For example, for Physics.SE the current meta post with the ads is: Community Promotion Ads - 2017. (To find it on any site, go to its meta and browse the community-ads tag.)
In there anyone can publish something which is related to the site, after carefully reading the rules, and when it gets enough votes from users, it will show in the sidebar in rotation with other ads.
Apart of that, the only place where you can have links to tutorials and other websites is your profile.
If your only goal is to post the link, then as Shadow Wizard has answered, you can use an ad or your profile. But if you just wish that everyone knew how to X, and you've written a tutorial about it, then you can take the advice in How can I link to an external resource in a community-friendly way?
You come across (or search for and find) a question (ideally without good answers already) that is about some subset of X. Let's say it's "why do I get error Y when I try to do x3?" You answer the question. Not "you have some config entries wrong, see my tutorial at link". A nice helpful self contained answer. Show exactly what the error is and how to fix it. Explain the misconception. Whatever. Then if you wish, you can end with "more details and screenshots [or a video, or sample code, or whatever] are in my tutorial at link.
If the answer is entirely useful without ever visiting the link, and if the link on the other end is nonspammy and useful (in my experience, it doesn't have to be free, but if it is a book you're selling or course that is for subscribers only, it's best to mention that so that people know before they click) and identifies the author, you should be fine.
But again, look to your motivation. Searching for questions so you can add short cryptic answers that promote your tutorial will lead to grief. Genuinely answering, and offering a link for even more information, will generally be ok.