When you first create a discussion, you have the option to make it a graded discussion. Making it a graded discussion will add it to the Canvas gradebook and make it so that you you can use the SpeedGrader to grade it.
From the Discussions page, click the + Discussion button
Under Options, click the checkbox next to Graded
Add other options
You now have the option to add points possible, display the grade in the desired format, choose an assignment group, assign peer reviews, assign to specific people or the whole class, and choose due dates and availability dates.