If there are no asset groups already set up, navigate to the Assets screen and click one of the purple "Create Asset Group" buttons to get started.
If there are asset groups already set up, navigate to the Assets screen (1), click the Asset Groups dropdown menu (2), and select one of the "Add" options (3) (depending on how you wish to go about it, for the sake of demonstration, we'll select "blank")
Whichever one you do, you'll find yourself on this screen. You're only obligated to enter a name for the group, but fill in as many fields as you feel are necessary (1), then click Save (2).
NOTE: Default depreciation methods/rates set here at the Asset Group level will be applied to any new assets you add via the interface. These default values are easily overridden on an asset-by-asset basis OR when you import in bulk via a file import with different values.