System memory and hard drive storage space are two different things. You need to be more specific. Is your system saying it is out of memory (system memory) or when you go save a file you get an error that says your hard drive is full (hard drive storage)?
Hard drives do not store memory. It stores data. If the hard drive is getting too full then you can delete files that you don't think you need anymore. You may also want to uninstall programs that you are no longer using. Lastly, you can buy an external hard drive to move data stored on the laptop's hard drive and on to the external hard drive. Simply select the files or folders you want to move, then right click and "cut" it from your laptop's hard drive and then open a window for your external hard drive by clicking on the "Computer" icon on the desktop. Then you can right click in that window and then select paste.
Alternatively, you can have one window open with the files you want to move and a window for the external hard drive. Select the files / folders and drag them to the window for the external hard drive. This only copies the files so after it is done then you need to delete the files / folders from the laptop's hard drive.
You can download a program called CCleaner which helps you delete a lot of temporary files that you really do not need to help clean up space. Every time you surf the net small files are written to your hard drive and they can accumulate over time.