One great thing about a wiki is that you it's easy to contribute. You don't need to set aside a day or even an hour to start contributing. You don't even need to register! (Though it's best if you do.)
Spending five minutes writing down something in your area of interest or expertise, or about a project that you know of, is a great way to start. Being a wiki, you can come back anytime and add another sentence, paragraph, or section. Others can also add to it.
You don't need to worry about formatting, or categories - all you need to do is start.
Starting with a new page? Click the Create a page link (always there on the left of the page), enter the topic name into one of the boxes, and click "Create page".