Accounts on the GitLab instance are tied to Wikimedia developer accounts. For help creating a developer account, visit Help:Create a Wikimedia developer account.
Once you have a developer account, visit gitlab.wikimedia.org and click "Sign In" in the upper-righthand corner of the window. This redirects you to idp.wikimedia.org to enter your credentials, and then back to the GitLab instance once you've signed in.
In order to prevent spam and abuse, newly registered GitLab accounts must be approved. This may be handled by the GitLab Account Approval Bot, but you can file an unlock request to expedite access.
Add an SSH key
editVisit the SSH settings in your profile and add a public key. See the GitLab documentation for more details on this process.
Add two-factor authentication
editYou may be required to use two-factor authentication to access most projects on GitLab.
Click on your user icon, then "Edit profile" or "Preferences", followed by "Account", and the "Manage two-factor authentication" button. From here you can set up an authenticator app, or use a hardware device (such as a YubiKey) as your second factor.
See the upstream GitLab docs on two-factor authentication for more details.