While logged in, click the "Assign myself an article" button in your "My Articles" section on the Home tab. This will bring up a list of articles your instructor has selected. When you find the one you want to work on, click Select to assign it to yourself.

Alternatively you can select an article by viewing the list at the "Available Articles" portion of the Articles tab and clicking Select. If you are working in a group, be aware that only one group member can select the topic via this method; the remaining group members will need to assign the article via the Home tab.

Review page 6 of your Editing Wikipedia guidebook for more tips.