Is there an article you have written and would like to import into Wikipedia? There are several ways to accomplish this.
The most obvious thing to do is to copy and paste content. Note, however, that most formatting will not be preserved. Also note that single linebreaks are ignored by Wikipedia, so if your source uses them, you will get very long lines. You can double all line breaks to replace them with full paragraphs. If you use Microsoft Word, note that in the replace feature ^p is the code for a linebreak, so you can replace ^p with ^p^p. Editors that support regular expressions may require a command like s/\n/\n\n/gs, respectively. Also note that Word uses so-called "smart quotes" (that look “like this”) which may be inadvertently included in your article. These quotes are generally frowned upon and do not display correctly in all browsers. One way of removing them is copying your text into another word processor like Notepad before pasting into Wikipedia.
If you want to properly import articles, you may consider using a conversion utility. David_A._Wheeler has written such a program for converting HTML to Wikipedia, see the html2wikipedia homepage. If you are unable to install and operate this program, just ask on this article's talk page, and someone may be able to help you getting your files imported. If your article is not in HTML, there are in most cases ways to convert it into that format.
If it is a very long article, and intended primarily for reference purposes, you can upload it as a file (link to it using the [[Media:Filename]] syntax. Since Wikipedia is not intended as an archive for source material, this should be used sparingly.
Please note that you cannot import articles which the author has not put in the public domain, under the Creative Commons Attribution/Share-Alike License (CC-BY-SA), or a compatible (i.e. weaker) license. Everything is copyrighted "by default" according to international law. Simple permission to "use" an article will not be enough, you have to make clear to the author that their work can be modified and distributed in accordance with the CC-BY-SA license.
Importing many articles
If you want to import many articles, it is usually a good idea to ask first if the material is appropriate for Wikipedia, for example, on Wikipedia:Village pump. If there is a very high number of articles, you may also want to consider writing or using a bot (i.e. a script) to import them; see Wikipedia:Bots for guidelines. Over 30,000 articles have been imported from US Census data, so it can be done.