[c#] C# referencing a variable from another method

I'm new to C# and i really need to know how to call/use a string from another method.

For example:

public void button1_Click(object sender, EventArgs e)
{ 
    string a = "help";
}

public void button2_Click(object sender, EventArgs e)
{
    //this is where I need to call the string "a" value from button1_click 
    string b = "I need ";
    string c = b + a;          
}

So in this example I need to call string "a" defined in function button1_Click() from function button2_Click()

Thanks!!