From what you've written, you're new to programming, and writing such a script would be beyond your current capabilities. You have several options:
(1) Unity has a built-in SmoothFollow.js script in Standard Assets/Scripts/Camera Scripts.
(2) The Unity Wiki at http://www.unifycommunity.com/wiki has several "improved" versions of SmoothFollow.js that may better suit your purposes. Read their descriptions.
(3) You could improve your current programming capabilities by learning Unityscript, C#, or Boo (your choice). Google "tutorial" plus one of those names... or check your local bookstore.
(4) Your last option is to hire a programmer (like me!) to do this for you. If you happen to be a good modeler or animator with samples, you may be able to barter.