If you are unfamiliar with wiki-editing, take a look at Help:How to edit a page. It is a concise list of technical guidelines to the wiki format we use here: how to, for example, make text boldfaced or create hyperlinks. Feel free to practice in the sandbox. If you would like a slower introduction we have a short tutorial.
These links may help you familiarize yourself with Wiktionary:
Entry layout (EL) is a detailed policy on Wiktionary's page formatting; all entries must conform to it. The easiest way to start off is to copy the contents of an existing same-language entry, and then adapt it to fit the entry you are creating.
Check out Language considerations to find out more about how to edit for a particular language.
Our Criteria for Inclusion (CFI) defines exactly which words can be added to Wiktionary; the most important part is that Wiktionary only accepts words that have been in somewhat widespread use over the course of at least a year, and citations that demonstrate usage can be asked for when there is doubt.