Once activated from the main Drupal menu, you can link an existing Lingotek account or create a new one. After it's activated, you can configure and manage the Lingotek module from the Translation tab under Manage.
You will also want to choose a few default settings. For example, decide which languages to translate into and which types of content to translate, which can be set in the Manage section under the Translation tab.
You can find more information on enabling languages, content types, and the translation process here: Getting Started (Drupal 8)