I presume you mean challenging a law on the grounds it is unconstitutional?. You would have to take a case to the High Court. You will have to prove that you have sufficient interest in the point being raised, ie, that the legislation is impacting you and also that you have grounds for your case. Decision can be appealed to the Supreme Court.
I'd expect this would not be cheap so you would need good independent legal advice, not the notions of pressure groups or other such like that have sprouted up in recent years