For .com and other generic domains, you can buy them from a certified registrar (list [verisign.com]) or reseller. For a country-specific domain (.ca, .fr, .co.uk etc.) then check with the registry for that country which companies are approved registrars.
Hope that helps. :)
However, when I go to the .net or .org, nothing comes up. How would I go about obtaining those?
Sorry I'm new to all this lol.
[edited by: encyclo at 12:55 am (utc) on May 14, 2007]
[edit reason] removed specifics, see your sticky mail :) [/edit]
After you perform a domain search (whatever you like) choose the domain you would like (if it is available) to get and simply complete steps in company's web site. You will endup with payment screen. As soon as you complete the payment process you will be the new owner of the domain.