When you are ready to send out your form, click on the preview icon (the eyeball) at top of the screen.
This will take you to a live preview of the form. You can then copy the URL from the address bar and give this URL to your students on the class page, or as an assignment, in an email, etc. (If you are going to do an email - it's a good idea to make a hyperlink instead of pasting the long ugly URL. Click here to see how if you are unsure.)
You can also get the link by clicking on the "Send" button across the top of the screen.
Clicking "Send" will take you to a window with a variety of options for distributing your form. Be sure to click over to the link section by clicking on the chain link icon. Then you can click "Copy" to copy the URL to your clipboard. You can then go and paste it where ever it needs to go (Classroom webpage, assignment, email, etc.).