i need to back up my macbook pro HD that wont start up

My macbook pro was being super slow so I turned it off using the power off button I then turged it back on but it got stuck on the white screen with the apple logo. I left it for hours and still nothing. I have tried a few steps I found on some forums but my computer seems to be providing different results of which it seems there is a problem with the hard drive. my mac has given me a few problems recently and think I am going to have to face buying a new hard drive but how can I back up the current hard drive (I have an external one)...There are really only 4 small documents I really need on there