Set focus to a textbox on page load

[Water Cooler v2]

I am new to ASP/ASP.NET so kindly be gentle. When my index.aspx page loads, I need the focus to be on one of the textboxes called txtUserName.

I come from a VB background, so expecting the obvious, I go to the Page_Load event of the index.aspx page and try to write txtUserName.SetFocus but I see there isn't a SetFocus method for the System.Web.UI.WebControls.TextBox class.

What's the way to set the focus to a textbox on the page load in ASP.NET?