To create a rank go to Administration panel=>Users & Groups=>Ranks=>Rank Administration and click on the "Add new rank"button.
Next you will see this page:

Spoiler:

In the "Rank Title" field type the name for the rank.
If you want a special rank choose Yes from "Set as Special rank" option.
If you want a normal rank choose No.
The difference between normal and special ranks is that normal ranks are automatically given to any member that achieve the number of posts required for that rank, while a special rank can only be assigned by an administrator.
If you make a special rank leave the "Minimum posts" field empty.If you want to make a normal rank type the minimum number of posts you want for that rank. Once that number is reached by a member he will automatically receive the rank.
In the "Rank Image URL" box you can add the direct link for the image you want as a rank. If you don't have any custom images at the moment you can choose from the default ones.
When you're done click the "Save" button.

If you want to add a color for your rank name you can add this HTML code in the "Rank Title" field:

Code:

<font color="red">Rank name here</font>

Replace red with any color you would like or with any Hex code color and Rank name here with the name of your rank.