My site uses the format domain.com, yet if I type that in to the address bar, it redirects to www.domain.com. I've read that this isn't good, and I want to redirect the www version to the URL w/o it.
My question: do I have to buy the www domain also to do so?
No, you do not need to purchase a new domain to do this. Domain registration includes both the www and non-www version of the name. You can redirect the www version to the non-www version using .htaccess.