Writing reusable c# code for ASP.NET websites

I have been learning C# for the past few days for use with ASP.NET to create websites.

I am very new to C# but I have been thinking about how I should go about writing my code to make it as reusable as possible.

As a quick example, lets say I wanted to create a piece of code to check a users login details which I could just drop into another site at any time, and have it work with the data it gets given.

Remembering that I have no idea how I should layout my code to do this, this is the idea I came up with (I will keep it short with some kind of pseudo code):

First I create a class:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;

namespace Module {
    public class Login {
        public static bool check_login(string usernameCheck = "", string passwordCheck = "") {
            if(usernameCheck == "user" && passwordCheck == "password") {
                return true;
            }

            return false;
        }
    }
}

Then I would have an aspx page where the login form would go, for example:

<asp:Content ContentPlaceHolderID="column1" runat="server">
    <asp:TextBox ID="usernameInput" runat="server"></asp:TextBox>
    <asp:TextBox ID="passwordInput" runat="server"></asp:TextBox>
    <asp:Button OnClick="check_login" Text="Login" runat="server" />
</asp:Content>

And the code behind file would look like this:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

namespace Module {
    public partial class _default : System.Web.UI.Page {
        protected void Page_Load(object sender, EventArgs e) {

        }

        protected void check_login(object sender, EventArgs e) {
            if(Login.check_login(usernameInput.Text, passwordInput.Text)) {
                Response.