C# - Properly Implement User System For Sessions

I've never actually implemented a registration/login system before, so I'm trying my hand at making my own in C#/ASP.NET (not using ASP.NET's built-in membership provider). What I'm a little unclear on is how to utilize Session/cookies to keep a user logged in during and between sessions.

protected void Login_User(object sender, EventArgs e)
{
string username = usernameField.Text;
string password = passwordField.Text;
User user = UserRepository.FindUser(username);
if (user != null)
{
if (user.Password.Equals(Hash(password)))
{
// How do I properly login the user and keep track of his session?
}
else
Response.Write("Wrong password!");
}
else
Response.Write("User does not exist!");
}