You can either declare Public Property in User control
Public string MyProperty {get;set;}
and access it like
myUserControl.MyPorperty="something";
where myUserControl is instance of you User Control
or if it is just for once
declare it in your constructor like
public partial class MyUserControl : UserControl
{
public MyUserControl(string myString)
{
}
}
and initialise you User Control like
MyUserControl myUserControl = new MyUserControl("someting");