Ensure you understand the philosophy of ArchWiki and think about what others may want to read (see ArchWiki:Requests for ideas). As mentioned, the wiki's scope is quite wide.
Talk to the admins for help coordinating major projects.
Help:Editing#Creating pages covers the steps to create a new article in the main namespace.
Consider creating a draft before publishing new pages to the main wiki namespace
If you are unsure whether a new article fits the philosophy of ArchWiki, this allows asking other users for opinions.
The wiki's subpage feature (article/subpage) is available for your userpage as well. Using it you could create the draft on a blank subpage. Creating a new user page is simple: Go to https://wiki.archlinux.org/index.php?title=User:YOUR_USERNAME/My_new_page to create a new page. Don't forget to replace the username in the link.
Please note that userpage content must not be categorized, but all other features can be used. Once you have finished drafting, you can simply move the subpage to its new name. Please add relevant article status templates (only) after you moved it. The better you describe a status template's reason, the easier it is for users #Improving the wiki to collaborate with you.