The first step is knowing when your opponent will be or is throwing his jab. It is different for every person but most people will have a 'tell'. A tell is something they do when they are about to throw a punch. Some will raise their elbow, some will turn their shoulders slightly, some will take an exaggerated step forward, some will stop moving completely first, these are just a few tells. You will have to become adept at spotting these on your own.
Catch The JabThe simplest way to negate the effects of an opponents jab is to block it, better yet to "catch it". To execute this simple maneuver you need to see it coming first, place your hand in the path of the punch and turn your palm towards it. This is similar to the way a back catcher in baseball catches a ball. Pairing this up with a jab is a very helpful counter punch. Catch the jab and immediately counter with your own jab into the opening created by your opponents attack. The countering jab should be thrown as soon as you detect your opponents jab coming in. When done correctly your opponents jab will be caught by your right (orthodox) as your own jab lands. A southpaw can catch the jab with his right and counter with a straight left.
Parry The JabThe next way to negate your opponents jab is by parrying it. A parry is like a catch but with a twisting of the wrist to knock the incoming jab down, this takes his jab away and creates an opening. Parrying with your right hand you can then come over with a straight right. This is a powerful way to counter. I myself find I tend to arm punch with this counter. The reason for this is I have a tendancy to turn my hips as I parry, try not to do this, instead let the punch come to you instead of you coming to it, its coming to you anyways whether you like it or not. Then after knocking it down fire your right.
Slipping The Jab
The next way that will take much more practice is slipping the jab. When slipping a jab slip to the right, slipping this way will move your head away from the opponents power hand (orthodox). Slipping the other way can and will eventually be devastating, opponents next shot will 90% of the time be from the hand you are currently moving your head towards. Moving into a punch effectively doubles its power. Slipping to the right and simultaneously ducking will bring you into position to either jab (a step to the right at the same time is often a good move) or throw a straight right into your opponents solar plexus.