The truth is, there are many different counters. If you are a lefty, you can use your straight left hand, you can use the left hook over his right, you can even step inside and counter with a right uppercut or right hook but it's more dangerous and risky.
The thing is, it should be something you work on and get good at because as a lefty, you are going to see right handed fighters more often obviously. The important thing is to stay defensively sound, keep your opposite hand up and learn to time that shot.
Easiest one is parry/catch the right hand with your left hand (back hand) and throw a nice crisp, short right jab. Try that was well.