It's very easy to edit the contents of a wiki. It only takes a few clicks.
When you need to use some type of formatting e.g. new headings or bold text, you do this using wiki syntax or the buttons in the edit toolbar. See Help:Formatting for some of the common types of formatting used.
Other people can correct mistakes later, so have confidence, and know that it is nearly impossible to "break" a page by editing it. An edit might be to contribute whole paragraphs or pages full of information, or it could be as simple as fixing a typo or spelling mistake.
Before you save a change, you can enter a short note in the Edit Comment box describing your changes. Don't worry too much about this, or spend too much time thinking about it, but try to give a little description of what you just changed e.g. "fixed typo" or "added more information about sunflowers".
The summary gets stored alongside your edit, and allows people to track changes in the wiki more effectively.
It's a good idea to use the "Preview" button to see what your change will look like, before you save it. This is also related to tracking changes because every time you save, this is displayed to others as a separate change. This isn't something to worry about too much, but it's good to get into the habit of eliminating mistakes in your own work, by using a preview before saving, rather than saving several minor corrections afterwards.
Another option is the "Show Changes" button which allows you to see the differences between the current version and your edited version.
With wiki edits you can start a new page, move (or rename) a page, or even delete a page: