Using Session is easy:
Session["MySessionVariable"] = myVariableValue;
...
myVariableValue = (MyVariableType) Session["MySessionVariable"];
So all you have to do is put a "next" button on you page, and in the Click event handler set the Session variables before you move to the page. When you load a page, yopu read teh session variables back in the Load event, and set the textbox content appropriately.
Because Session holds object values, you can store whatever type you need.