Using Selenium Webdriver in C#, how do I select a text box to write in, then write in it?

You will need to give us some HTML of the page, but given a password textbox like this:

<input type="password" id="passwordTextBox">

I'd find it using Selenium's WebDriver like so:

IWebDriver firefoxDriver = new FirefoxDriver();
IWebElement passwordTextBox = firefoxDriver.FindElement(By.Id("passwordTextBox"));

I would then 'write' into it like so:

passwordTextBox.Clear();
passwordTextBox.SendKeys("password");

I would take a look at the Selenium Web Driver documentation, and ask any questions after you've read through it all:

http://seleniumhq.org/docs/03_webdriver.html