First you have to locate the web-element uniquely presented as select list on UI, you can use Firebug for that to find list element's id, name, xpath or css path. Then create SelectElement object passing this element locator attribute. To select an option in the list, you can choose any option like SelectByValue or SelectByText.
using OpenQA.Selenium;
using OpenQA.Selenium.Firefox;
using OpenQA.Selenium.Support.UI;
class SelectingOption
{
static void Main(string[] args)
{
IWebDriver driver = new FirefoxDriver(); //Launch Firefox browser
driver.Navigate().GoToUrl("http://www.abc.com/"); //Go to application URL
//Authentication and navigation code to page where select list there
SelectElement se = new SelectElement(driver.FindElement(By.id("select_element_id"))); //Locating select list
se.SelectByText("Item1"); //Select item from list having option text as "Item1"
//se.SelectByValue("Item1"); //Select item from list having option value as "Item1"
//Following code to selecting item from list
driver.Quit(); //Quitting the Firefox
}
}