How do I free up my MacBook's Drive Space?

For the first time while trying to import a CD onto iTunes, I got a popup about how “my start-up disc” is almost full, or something like that. In my Macintosh HD I have 59.81 GB capacity and only have 3.18 GB left, that kind of worries me. I’ve only had this laptop for about two months. I am not sure what used up all of my space so quickly, I don’t have many picture and I only have 2 extra applications (uTorrent and VLC). Do you think the fact that I downloaded 2 movies and 1 TV season (which was 10.8 GB) is what took up all of this space?

What simple things can I do to free up some space? I have heard about something like sudo periodic daily, weekly, monthly. If it is possible, please explain how to do whatever you suggest if it is complicated and what it will delete on my laptop.

Here’s an bonus question: If I convert this TV season onto my iTunes, then move the download to my trash, will it still be taking up the 10 GB on my laptop? Is there any way, other than putting it on a CD that I can free up the space and still be able to view it? If I import music/movies/pictures onto my iTunes, then delete the file on my downloads folder, will it delete on my iTunes?