how to fiil data in usercontrol textboxes

You can use properies to set and retreive a private variable. Since Textboxes are private objects , here also you can use properties .
In user control writ a property to set the value of the textbox

public string textValue
       {
           set
            {
             textBox1.Text = value;
            }
           
           get
             {
             return textBox1.Text;
             }
           
       }

You can access this property at you form by calling

userControl11.textValue = "the value for user control.";

You will get the user control value by

string userctlvalue = userControl11.textValue;

Hope this will help you :)