You can use WebDriverBackedSelenium class to convert WebDriver interface into Selenium interface:
Selenium selenium = new WebDriverBackedSelenium(webDriver, baseUrl)
;
or you can use the method getUnderlyingWebDriver() to convert Selenium interface into WebDriver interface:
WebDriver webDriver = ((WebDriverBackedSelenium)selenium).getUnderlyingWebDriver();
I posted slides of one of my recent presentation on how to get started with Selenium 2 here:
http://www.slideshare.net/sebarmeli/getting-started-with-selenium-2
One of the best practices in Selenium 2 is the PageObject pattern.
If you are using Java and Maven, you may want to check out this Archetype plugin (that also gives you a first taste of the PageObject pattern):
https://github.com/sebarmeli/Selenium2-Java-QuickStart-Archetype