This can be as simple as
MyTextBox.Focus
. It will work, first of all, because you are asking to do it on button press, when everything is rendered and shown on screen. But in many cases you may need to solve more sophisticated problems of focusing, which is not a trivial WPF topic:
http://msdn.microsoft.com/en-us/library/aa969768%28v=vs.110%29.aspx[
^].
I don't know why would you need a "particular blog" on this. Original documentation is usually enough.
But first of all, I would advise to review your UI design. Focusing something on button click sounds weird.
—SA