How can I start and stop my countdowntimer via a button?

Well... maybe you need to first understand how Java and programming work. Then, you can try to do something like this:

CountDownTimer aCounter = new CountDownTimer(100000 , 1000) {
    public void onTick(long millisUntilFinished) {
        mTextField.setText("Seconds remaining: " + millisUntilFinished / 1000);
    }

    public void onFinish() {
        mTextField.setText("Finished");
    }
};
aCounter.start();