Help:Introduction to the Manual of Style/1
Making articles readable
Images and refs
Enriching the text
Tying the encyclopedia together
Final little things to think about
Review of what you've learned
The Manual of Style (MoS or MOS) is an in-depth guide that provides standards on how to format Wikipedia articles. Following these guidelines helps keep the encyclopedia clear, consistent, and stable.
The simplest way to do this is to find a well-written article and copy its formatting. But if you want more information on any formatting, the MoS probably has a recommendation (you can search its contents here).
Remember, the MoS is a guideline; you don't need to have the whole thing memorized! It's there to assist you when you're unsure how to best display information, and to minimize arguments if another editor disagrees with your formatting choices.
Content is more important than formatting, and other editors can assist you if you're in doubt (similarly, assume good faith when others help by formatting your writing).