Take time learning the secrets of buying and selling domains. Just dont go registering random domains. Try to keep it under 10 characters.. make it brandable. The easier the name is to remember the better off it is. The Domainers Blog
Also try to buy old domains, but like Desilator said, try to keep them short and/or meaningful. Be really careful of buying "PR" domains. Make sure they have a valid pagerank, and if so, are not "dropped" domains, as most likely the PR will dissapear with Googles next update. You can tell if they are dropped by checking domain age at checkpagerank.net. If it's a PR3 or PR4, and it only shows as 3 months old, it's a sure bet it's a dropped domain and the PR won't hold. GoDaddy has "close out" auctions (TDNAM.com) where a domain has expired, but not dropped, and you can buy them for 5.00 plus registration fee. There is a lot of junk there, but you can find some good aged domains, domains with PR1, PR2, and PR3 there as well, and they will hold their "age". No guarantees they will hold their PR, though.
the best place to start is with the register function at your favorite registrar. when your search for a domain comes up "taken" "registered" --> make your search generic enough to allow the registrar's suggested alternative names to display. make sure the name is failry short and easy to spell and not trademark infringing and start hand registering some names.