Get DOI for publication
As others have noted, the difference between ISSN (International Standard Serial Number) and DOI (Digital Object Identifier) is that ISSN identifies a periodical publication (i.e. journal, conference proceedings) as a whole (i.e. it does not change between volumes or issues), while DOI identifies a single published work (i.e. article) from that periodical (more specifically, it also identifies a version of that work).

Most journals provide a DOI for their publications, even if it is not immeadeately obvious. Typically it's somewhere close to the top of the page while browser-viewing the article.

To obtain a DOI from a reference, take a look here. It seems to be an official service from Crossref that will search for a DOI based on a bibliography entry.

So, you just enter text in there (I tested the service by copy-pasting the first 7 bibliography items from the reference list of my latest publication), one reference per line, click "submit" and it will try and find a DOI for each entry. Out of the 7 references I submitted, it succeeded for 6, while the last one was a book and did not have a DOI.

Note that not finding a DOI through the suggested link still does not guarantee that a publication does not have a DOI (but some do not); in that case the next-best options are to check the journal website as well as how other papers cite the work in question.

Finally, it was suggested that posting a preprint of a paper on ResearchGate will allow you to generate a DOI for it. This is true, but with one caveat - as mentioned, the DOI identifies a single instance of an article, and therefore the DOI generated through ResearchGate will be linked specifically to a preprint (or "author's version"), and not the version of the work published by a journal or in conference proceedings.