The following is the snippet of WebDriver code using Java:
WebDriver driver = new FirefoxDriver();
driver.get(\"http://www.google.pl/\");
In this case the XPath expression you want is:
//html/body/center/form/table/tbody/tr/td[2]/div/input
Or you could use this (a little more intuitive):
//input[@title='Google Search']
Keep in mind that if you will be identifying a lot of elements by XPath it would be advisable to become fluent in XPath, you could start here: Xpath Tutorial
In the meantime, use Firefox and install the following plugins:
Firebug
FirePath or Firefinder
These will help you easily identify valid XPath expressions to use for your website.