Actually both but your content is way more important than the domain. The idea is to choose a main keyword phrase you'd like to target and build as much content around that phrase (and related phrases) as possible.
This takes time and you won't see traffic immediately but the more useful your site is to your target audience, the better.
First I would decide what interests you. Then I'd do some research on some keywords related to that subject to see what and how people are searching for that subject.
Go to
www.selfpromotion.com to learn more about how search engines rank sites. It's a good overview.