Quickest way to get a landing page
For the content: Use an approriate content type or view.
For the URL use an URL alias to get something like www.mysite.com/landingpage .
If you want to hide some blocks on the landing page add a PHP snippet in the block configuration which queries arg() or $_GET['q'] for the URL.
If you need an other theme have a look at the ThemeKey module.
Use the Nodewords: D6 Meta Tags module to add meta tags for SEO or Facebook open graph tags.
Use the statistics or Google Analytics module to track the succses of your page.