How do I move my iTunes account onto a new laptop?

I owned a Dell Inspiron in which I have my iTunes account set up on. Today I bought a MacBook Air and I want to transfer my iTunes account onto my new laptop. I read something about de-authorizing or something but I am not sure how to do it right. I absolutely DO NOT want to lose all my music and movies because I have lots and spent a lot of time getting all of it. I also am worried that it will only save the media I have purchased, which isn’t much. How can I transfer my iTunes account onto my new laptop without losing anything?