Visit the Drupal Core Mentoring site task tracking tool, and create an account if you don't have one already.
Pick a task with the green "Yes" column at left. This indicates that no one else is already working on the task. Click the title in the Task column to view the task and decide if it's one that you can help make some progress on. If so, click the "+ Add" link below the "Task Attempt" header, to indicate that you're going to work on the task. Your name will be filled in for the Participant field; just click Save.
Next, visit the issue on drupal.org and read the issue and comments to see how far along the task is. You can get to the drupal.org page by clicking on the d.o nid number.
Reproduce the current state of the issue and see if you can move things forward. If so, certainly post a comments of your own.
On the day of the issue sprint, pair up with the Mentor listed on your chosen task, and work together to move the issue forward in some way, either by adding new information to the comments, or by testing a patch someone else has posted, or by writing a patch of your own.