If you add good material to an article, cite it in any of these ways:
Post any citation format into the article without any regard for good Wikipedia practices. If you do it incorrectly then someone will fix it or tell you or both. This is least preferred and you should not make a practice of it, but if the Wikipedia interface - which is supposed to be self-explanatory - still seems troublesome to you, then do this.
If editing an article, then at the top of the text editing box look for the "Citation" tab and the "templates" box. Pull up a citation template, such as the one for journals, then complete the fields. If you have a DOI then just paste that into the box and that will generate a full citation.
There are other templates you can use if you remember them, such as by typing {{cite doi|12345}}
Since any part of Wikipedia can be edited by anyone, Wikipedia articles are typically much more heavily cited than other publications. Feel free to put a citation at the end of every sentence.