To create a Gradebook, click on the Assess Tab from inside your class, and then click on the Gradebook link (directly underneath). 

A drop-down menu will appear with any existing gradebooks listed first (if any) and an Add Gradebook button. 

Click Add Gradebook.  A window will pop up, asking you to name your Gradebook and whether or not you would like to use Gradebook Categories. 

Gradebook Categories allow you to create groupings within your gradebook, such as "homework," "tests," "projects," etc.

You are also asked to what decimal you would like to round your grades (one decimal, two decimals, or strictly whole numbers) and how you would like your grade subtotal displayed (by percent, letter grade, or both). 

You may also select a Grade Notation Set and a Special Score Notation Set* from the default options, or select Create New and create your own grading scale on the next window to pop up. 

If you choose to create your own grading scale, you may name your Notation Set and then set the Notation, what a given percentage translates to in terms of notation, and what specific percentage the gradebook should translate any grades entered into the Gradebook in Notation form.

**Hint: Special scores are used to notate gradebook entries such as Excused, Inexcused or Incomplete. These notations are used to award 'no credit', 'full credit' or to ignore the entry in the final grade calculation.

If you'd like to set up Terms and Grading Periods, you can do this in the Class Schedule (Manage Class > Class Schedule). 

If you'd like more information about setting up a class schedule, you may find the following articles useful:

Setting up a Class Schedule using Trimesters as an example:
http://support.haikulearning.com/entries/20680133-t-how-do-i-create-trimesters-for-my-gradebook

Creating a New Class:
http://support.haikulearning.com/entries/31073-t-creating-a-new-class