Well, as you state you are new to .net I am afraid there is no short answer to your question but maybe I can point you in the right direction.

Create a ASP placeholder on you page and on the SelectedIndexChanged event for the dropdownlist add a textbox and checkbox to it.

For example like this:

                TextBox mytextbox = new TextBox();
                mytextbox.ID = "txt1";
                PlaceHolder.Controls.Add(mytextbox);

Make sure you create a unique ID for each control which you will need when the time comes to read data from it. You can access the control by using 

TextBox mytextbox = PlaceHolder.FindControl("txt1");

In order for these dynamically added controls to maintain it´s viewstate you could use this control this is a special placeholder which maintains viewstate after postback so any values entered in them will be available after postback.

I know this is a very short description of what is needed but maybe it will help you in the right direction.

Stephan
Dotnetnuke Module