All you need to do is set two passwords (really only one is required) to get started editing pages. The site-wide author/editor ("edit") password enables page editing and the administrator ("admin") password enables site management.
Edit the local/config.php file with a text editor and put the passwords in the appropriate places (described in the file's comments).
While you're in there you can also set your website's name.
Uploads (attachments) are enabled by default because most people would want that for uploading image files. You can disable uploads if you prefer.
You can optionally encrypt the passwords in the configuration file. Visit your site with ?action=cypt and follow the instructions there.
When the passwords are set, your site is ready to use.
PmWiki is well-written software that makes it easy to edit your pages and create new ones. Just type some text and hit "Save". It gets even easier (and you can do fancier things) with a little practice.
If you want to go beyond Basic Setup you can do some new site enhancements.