Any answer should include what and why. What is the answer? Can you identify the exact work? And why is that the answer? Does it match the description given?
In the case of story IDs, the what should at least include the name of the work, the name of its creator(s), and the medium (was it a book, a short story, a (web) comic, a film, a television series, a game, and so on).
A rough indication of when it was first published, released, or broadcast is helpful as well, especially in the case where there are several works with that same title.
If it's a part of a series of franchise, naming that and placing it within that would help a lot too.
For a book or story, that means the title and the author, as well as a mention of what kind of work it is — series, book, novel(la), short story, or fan-fiction.
Film and television
For a film or television series, at least the title. Preferably the director and the main actors as well. Year of release or broadcast is a bit more important here than it is for books, since that helps narrow it down considerably, more so than with books that can often have several print runs and can be republished.
Indication of whether it was an adaptation or a remake can be very helpful as well — maybe you've seen the remake while the querent has seen the original, for instance.
For a comic, that should include the writer and the artist(s), the name of the series, and the number of the issue. Possibly the publisher as well.
At least the title and the system it was released on. Preferably the company that released it too, as well as the year of release.
This is as important as what. The querent has described a number of elements that may or may not match. Either give a synopsis, or address them point by point.
Don't forget to address the points that do not match.
Linking to a source such as Wikipedia, the ISFDb for written works, Goodreads for books, the IMDb for films and television, Rotten Tomatoes for films, is very helpful, but only required if you quote from it.
How is entirely optional, but a lot of people have searched extensively already. Telling them how you've found it might help them understand why they couldn't find it and may even help them searching for another title in the future.
Linking to the work in question
While linking to the exact work can be very tempting, it may be in violation of copyright. A trailer on YouTube or any other video site is probably fine, especially if published there by the (presumed) copyright holder, such as the studio. Full films can be found as well, but should not be linked to unless you can be reasonably sure that it isn't in violation of copyright. Some short and even medium length films have been released on YouTube, by its creator(s).
The same goes for written works and comics. If published online by the author or artist, it's fine. Some writers publish online, some do that for old works that are no longer in print. Web comics are of course published online.
The Internet Archive has a very large collection of old magazines, in which stories may have been published before being published in book form. They're a large enough entity to assume they know what they're doing copyright-wise.
Do not link to torrents, rips, scans and such that are clearly in violation of copyright and often hosted on sites of dubious reputation and security as well.