If something should happen that is warranty worthy or Apple Care worthy (like Hard drive failing or Ram not working properly) you simply take it to the Apple Store, or call 1-800-MY-APPLE and explain to them your problem.
They will arrange setup to start the process which will fix your MacBook. If you call Apple you will be sent a box in which to send your MacBook to Apple Care and they will do necessary repairs and return via Fedex. If you choose to go to the Apple Store, first make an appointment online to meet a Genius, explain to him and he will setup repairs to it or need be replace it.
Which ever case you go, Apple's quality service will be there to assist you.