We could help better if you report the error messages you receive. That way we could get some idea of what you are doing.
You mentioned that you "downloaded" Joomla!; I can see how that could be confusing if you are new to this kind of stuff. Joomla, like WordPress suggested by lensman, does not need to be downloaded (then uploaded and installed) at all on DreamHost - use the Control Panel's "Goodies - One-Click Install" Robot to "automagically" install these programs so you can explore.
I agree with lensman that WordPress would be a good starting place, as Joomla can be a good deal more difficult to get your head around (though it is arguably more powerful).
Wholly has the right idea too - take it a step at a time and explore, or seek some "one on one" help from someone who understands what you currently know and can help you over the initial "humps".