Help with a class

I am converting one of my plugins to Class instead of functions and I am encountering an error. PHP classes are fairly new to me so I am having some trouble.

I am getting the following error:
Fatal error: Using $this when not in object context in … on line 207

Here is the method inside the class:

public function getLocations(){

		$Locations = $this->db->get_results("select location_id, location FROM {$this->my_locations_db}");

		return $Locations;

	}

Line 207 starts with $Locations

I am calling the method from inside the class using:

// get all the locations
		$myLocations = self::getLocations();

I am calling this same method through another method without a problem.
I am getting the results I am seeking, however when I call it from a different method I get the error.

Your help would be greatly appreciated.

Thanks