Weblate will now present you with text in need of translation or correction. Confidently submit a translation, or fill-in your proposal and submit the form. If you're not sure at all, comment or move to the next string.
The Framasoft GitLab instance is hosted at framagit.org. You have to sign-up here, using your e-mail account or by way of a third-party token provider.
Telling Weblate what languages you understand/translate will make it easier afterwards. You can also take a moment to edit your account profile and settings if needed.