I have put my web site up on the internet and google has pretty much completely indexed it. I have changed my url structure so now all the pages google has indexed are 404. Is there a way I can get google to delete all these pages and then
You should be doing 301 redirects from the old URLs to the new URLs. That way Google knows the pages have moved and associate the new URL with the old URL.
But, if for some reason you don't want to do that, don't do anything. The 404 errors will cause Google to eventually stop crawling those old URLs. The new URLs will be found through internal links on your site and through your XML sitemap. You did submit an XML sitemap, right?