To add items, click "Items" on the left navigation menu, then the green "Add an Item" button. You will be prompted to create metadata (or descriptive information about a digital object) about the item you are uploading. Which fields need to be filled in will vary based on your class assignment; check with your professor if in doubt.
When you are done entering the descriptive information, click "Files" at the top to upload your file. Finally, click the green "Add Item" button on the right to publish your item. By default, items remain private from public view until you publish them. Click the "Public" checkbox under the green "Add Item" box in order to make an item publicly visible.
Hover over the yellow stars in the image below to learn more about the different metadata fields.