You can add a citation by selecting from the drop-down menu at the top of the editing box. In markup, you can add a citation manually using ref tags. There are also more elaborate ways to cite sources.
A "citation needed" tag is a request for another editor to verify a statement: a form of communication between members of a collaborative editing community. It is never, in itself, an "improvement" of an article. Though readers may be alerted by a "citation needed" that a particular statement is not supported, many readers don't fully understand the community's processes. Not all tags get addressed in a timely manner, staying in place for months or years, forming an ever growing Wikipedia backlog—this itself can be a problem. Best practice recommends the following:
Before adding a tag, at least consider the following alternatives, one of which may prove much more constructive:
Currently, there are over 430,450 articles with "Citation needed" statements. You can browse the whole list of these articles at Category:All articles with unsourced statements.
Frequently the authors of statements do not return to Wikipedia to support the statement with citations, so other Wikipedia editors have to do work checking those statements. With 430,450 statements that need WP:Verification, sometimes it's hard to choose which article to work on. The tool makes that easier by suggesting random articles, which you can sort by topical category membership.