correct way to create an object of the class?

AFAIK, there is no difference between the two.
On the first one, you just immediately instantiate it after you declare your object.
On the second one, you declare your object first, then you instantiate it on the next line.

I always use the second one ( which I really don't know why) maybe because you can instantiate it on which event you needed it to be instantiated. Example:

Sample s;

private void btnClick_click (object sender, EventArgs e)
{
//you don't need the object here yet..
//add code here
}

private void btnSubmit_Click (object sender, EventArgs e)
{
//in this event you need to instantiate your object.
s = new Sample();
//add code here
}