Hello Pepy and welcome to the community! 
To find and claim a task, take a look at these video tutorial and FAQs. However, I don’t see that Greek is available in the target language drop list, which means that there are no available tasks into this language for the moment. But you can still visit your home page another time to see if new tasks are uploaded. You can also turn on the email notifications for the task stream from your profile.
If you would like to have access to more tasks, I would recommend that you take a test to become a verified translator. Take a look at this thread: How to become a Kató Verified Translator?
Hope this helps! 