If you're ever contacted via email, text, etc and there's a link in the message, inviting you to click on the link to log in - NEVER click on the link.
If in any doubt, log in using your normal method, whether that be a saved website in your favourites list, or simply by typing the address into the address bar - that way, you know you are at the actual website that you want to be.
Most of these scams claim to be from either your bank, PayPal, iTunes or some other institution and they nearly always claim that there's a problem with your account and provide a link for you to log in to. Unfortunately you're taken to a fake website that looks like the real thing and you unwittingly log in - providing the scammers with your account details and password.
If in doubt, simply log in using your trusted method .... but NEVER click on any links :)