Simple text box, directions?

You can use the Unity UI system and EventSystems coupled with some custom scripting to create this logic.

GameObject > UI > Text - this will create a new Text element (along with a UI Canvas and EventSystem) and you can edit the displayed text (and change it dynamically with its text property).

Position UI elements with the RectTransform tool found in the Toolbar. In the Scene view, drag the Text element inside the (very large) Canvas rect so you can see it in Play mode.

Create a custom script (C#) - in the Insepctor > Add Component > New Script - give it a name, e.g. MessageBox. Then click the new component's gear icon > Edit

In the new MonoBehaviour class add the following code:

     public UnityEngine.UI.Text textbox;

     void Update()
     {
         // if player presses 'E' key this frame
         if (Input.GetKeyDown(KeyCode.E))
         {
             // if textbox reference exists
             if (textbox != null)
             {
                 // toggle textbox (visible/hidden)
                 bool wasActive = textbox.gameObject.activeSelf;
                 textbox.gameObject.SetActive(!wasActive);
             }
         }
     }

In the Inspector of the MessageBox component, assign a reference to the Text element by dragging it from the hierarchy into the Textbox property of the MessageBox component.

For more information, Google is your friend.