The most commonly used autoblog plugins are Autoblogged, wp-o-matic, Yaab, and some other one that I forget the name of. Which one is the "best" is hard to say, as what works well for some people may not work well for others. I've tried all of the ones that I mentioned, and I'm not ecstatic about any of them. If you're looking for a free solution, then wp-o-matic or Yaab will work. Autoblogged is a bit pricey but more user-friendly than the other two.
I suggest that you search the forum for the programs that I mentioned. Also check out the autoblog tutorials. There are many of them here.