I have been tremendously successful getting rankings with Joomla in very competitive areas. My formula is Joomla + SEF Advance (Sakic.net) + SEF Hack (free). These will convert all of your existing pages to SEF friendly URL's. Your pages will look like
www.yoursite.com/this-is-my-page. You can customize SEF advance and I recommend using - instead of _. Obviously, there are other factors in play - content, directories, backlinks, etc. Also, if you can, code your pages rather than using the WYSIWYG. Lastly, take a look at
SEO friendly templates such as the ones from Joomlashack.
Good luck! Don't get discouraged. Joomla is very
SEO friendly with a little tweaking.