There are some things to keep in mind when editing Wikipedia.
Wikipedia is an editable encyclopedia (along with some topics that would typically be found in an almanac). Hence, articles should consist of encyclopedic information about "notable" subjects. What exactly constitutes notability is the subject of constant debate on Wikipedia, but few of us believe that there should be articles about every person on the planet, every company that sells anything, or each street in every town in the world. However, there are sister projects for certain types of non-encyclopedic content.
Any article that simply defines a word, or short phrase, as you would find in a typical dictionary, and that cannot be expanded into an encyclopedic entry, should be contributed to the Wiktionary sister project.
Original source text, such as from a public-domain book that you want to post to make it more accessible, should be contributed to one of Wikipedia's other sister projects, Wikisource.
We also tend to discourage authors from writing about themselves or their own accomplishments, as this is a conflict of interest. If you have made notable accomplishments, someone else will write an article about you eventually. Wikipedia:Autobiography has more detail on this.
Neutral point of view
Wikipedia's editorial policy is the "neutral point of view," often abbreviated "NPOV." This policy says that we accept all the significant viewpoints on an issue. Instead of simply stating one perspective, we try to present all relevant viewpoints without judging them. Our aim is to be informative, not persuasive. Our policy does NOT mean that our articles are expected to be 100% "objective," since in any dispute all sides believe their view to be "true."
It is OK to state opinions in articles, but they must be presented as opinions, not as fact. Also, it is a good idea to attribute these opinions, for example "Supporters of this say that..." or "Notable commentator X believes that..."
You might hear Wikipedians referring to an article as "POV." This is Wikipedia slang for a biased article, or one obviously written from a single perspective. Advertising would fall in this category, as would a political diatribe. In a less extreme case, an article might have "POV" problems if it spends significantly more time discussing one view than another view of equivalent significance, even if each view is presented neutrally.
If you are going to spend time on controversial articles in subjects like religion or politics, it is important that you read the neutral point of view policy page as soon as possible. You should probably also read Staying cool when the editing gets hot. If you are going to spend your time on less emotional subjects like math or video games, you should still read the policies, but it is a less pressing concern. Keep in mind the advice here, and read the full policy if an NPOV issue comes up. See also the NPOV tutorial.
Wikipedia requires that you cite sources for the information you contribute. All sources should be listed in a section called "References". If any websites would be of particular interest to a reader of an article, they should be listed and linked to in an "External links" section, and books of particular interest should be listed in a "Further reading" section, but only if they were not used as sources for the article. Citations help our readers verify what you have written and find more information.
Do not submit copyrighted material without permission. When adding information to articles, make sure it is written in your own words. Remember that all information found on the Internet is copyrighted unless the website specifically states otherwise.
5. The usage should be consistent throughout an article, unless it mentions both US- and Britain/Commonwealth-related topics. In that case, Policies 2 and 3 prevail.
6. When you create a new article, generally the most commonly used title is preferred. A simple way of testing this is to try a Google Test either on the actual Google or on GoogleFight.com, or on a different search engine to your preference, and see which generates more results.
Wikipedia encourages an atmosphere of friendliness and openness. Of course, in practice there are sometimes disagreements and even an occasional heated argument, but members of the community are expected to behave in a generally civil manner.
The most important thing to keep in mind is that you should always assume good faith on the part of other editors. Do not assume that someone is acting out of spite or malice. If someone does something that upsets you, leave a polite message on the relevant article's talk page or on the user's talk page, and ask why. You may find that you have avoided a misunderstanding and saved yourself some embarrassment.
If you find an article that you believe is mis-named, please do not copy and paste the contents of the old article into a new article — among other things, it separates the previous contributions from their edit history (which we need to keep track of for copyright reasons). The preferred method is to move the page to the new name. If it is your first move, please read the warnings on the move page carefully, as there are a number of issues to consider before moving a page. If a "disambiguation" page is involved, it is best to review Wikipedia:Disambiguation.