When given an assignment, click on the assignment and you will be brought to this page. The assignment details are on the left-hand side of the screen. Make sure that you are in the right assignment and read the instructions carefully. You typically only get one submission per assignment so be sure that it is exactly as you want it before submitting it. The "Add Attachments" button is what you use to add files from your computer to your assignment.
When you add an attachment it looks like this:
Make sure you have everything just as you want it before hitting "Submit." You only get one chance.
You should see this page upon submitting your completed assignment: