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Yahoo will register your domains' names for under $10 a year, though as of this writing, Yahoo is having a sale on domains for $2.99 per year. That's a lot cheaper than some registrars I've seen that are still trying to charge $20 or more just for domains.
Once you get your domain registered, you still need to host it somewhere. Again, Yahoo has a simple answer, or several simple answers, in its Geocities service. You can plunk down your domains for free at Yahoo Geocities, but you'll have ads. For a nominal fee ($4.95 per month) you can get 500 MB storage and 25 GB per month transfer -- that's more than enough for most starter sites.