Click the link that reads PostgreSQL on the left bar.
Login using the group credentials that were given to you (do not forget to change your password).
Select the schema that you are going to use in the left bar.
Click the link that reads SQL on the top bar to open a SQL command window.
Some important notes:
Do not drop (delete) your database as you do not have permission to create it again.
Use the public schema for your project. Create a new schema for each exercise.
To create a new schema, click the schemas link on the left bar and then the create schema link on the main window. Each exercise is composed by a script and some questions. Start by creating a schema for the exercise and run the script on that schema so that the tables are created and populated.