If you are running out of room on your macbook than the video below will take you step by step on the process of upgrading your hard drive. In the video I use a 500GB hard drive for the upgrade. Below are also links where you can get this drive.
WHERE TO BUY: 2.5 SATA 500GB 7200RPM Drive
Other World Computing $149.99: http://eshop.macsales.com/item/Seagate/ST9500420AS/