First, your "health" has to be a class scope variable. Right now, it's scope is the constructor. Outside the constructor, no "health" exists.
If you want an event to occur whenever "health" is changed ( for example putting the new value in the textbox ) a property would be fine. The set method could set the textbox ( for simplicity ) or call an OnHealthChanged Event. But I guess that's a bit much, so as step one, declare "health" in the scope of your form like this
and move your textbox setting line into a function. A class can never have a line that is not a variable declaration outside of a function.
partial class Form1 : Form
private int health = 100;
textBox_health.Text = health.ToString();