Editing a WiKi is pretty easy and outside the scope of this forum (hit google), but the very basics of it as far as the NSIS wiki goes for your specific case:
Browse to e.g.
It will tell you the page does not exist and gives you a few options. Choose to 'edit this page'.
You'll then get a small web-based editing interface where you can write text, etc. Easiest for you may be to take an existing page, such as this one:
And edit that (at the top there's an Edit link), then copy the text there to the new page you're editing.
Adjust the text you pasted to fit your own plugin.
Go to..
..to upload your actual plugin file (preferably zipped up).
Remember the name of your file, and add..
<attach>yourFileName.zip</attach>
..somewhere in the page you're editing to create a link to it.
Finally, hit save - and voila, you've got a whole new wiki page of your own 🙂 You may have to register with the wiki before you can do things like upload files, but that's pretty standard 🙂