You need to work with a trainer experienced with dog-dog aggression. Ask your vet to recommend someone. In my opinion, this is not something to tackle on your own, particularly if you are feeling nervous and scared. You need someone to teach you how to train your dog.

Good luck.

Okay, couple of questions...
How long have you had the dog?
How old is the dog?
Is the dog fixed?
Do you have any information on its life before you got it.
Next - How often are you walking the dog and for how long?
Is your dog wearing a soft collar, harness or choke collar?
What are you walks like and how often do you walk your dog?
How often do you cross paths with another dog?
How do you start your walks - give me details right from the minute you are thinking of walking the dog, how you go about leashing up the dog to how you leave your house and how the walk d\goes when you dont see another dog...is your dog sniffing everything or is it under control?
ALSO - one more thing, how much obedience training have you done with your dog....
Please be as specific as you can in your answers.
Note - you do NOT need to give your dog up or put it down, with the correct training you and your dog can have calm walks that everyone can enjoy BUT you need to be dedicated to working things out.
Hope to hear from you soon.

Sounds like you just adopted your pup. Have you signed up for obedience classes yet? Find a good trainer who uses positive reinforcement (clicker training and treats). Talk to her about your dog's reactions to other dogs before the class. She may recommend that you work with your dog before introducing him to a group situation. It's not likely that your dog is truly 'aggressive.' This is more likely fear aggression.

You can countercondition the behavior by carrying a pocket full of little training treats on walks. When you see a dog far in the distance start treating your dog. Keep treating as you get closer and closer to the other dog. Don't say hello (cross the street if you need to) and stop the treats as soon as you're past the other dog. If you work this consistently your dog will start to associate seeing another dog with something positive (the treats.) Be patient, avoid situations which put your dog in the position of reacting, and never say 'NO'. 'No' isn't a command, and you can't put emotions on command anyway :) But you can teach your dog to associate other dogs with good things. It may take a little time, but it's worth it.

JR's should be six feet tall and weigh 300 pounds judging by their attitude.:)

The advice about training is good, but there are things you can (must) do as well.

First. Whomever walks the dog must be the dominate person(to the dog). All the training possible does not overcome that fact.

Second, training means getting the dog to understand what the trainer wants and then reinforcing that behavior. Makes no difference if it is Sit/Stay or Don't Bark. The best way, in my experience , to truly train a dog is to do so using methods the dog inherently understands. Reward training works very well for tricks and tasks(like agility training).

I do not use reward training for basic behavior training. Other than praise/affection, that is.

The problem with your dog is one of the more difficult ones to correct, as it is deeply seated in instinct that has been bred into that breed for decades or longer. JR's were bred to hunt small game---often animals that burrowed. So, a dog going into another animals safe place better be aggressive, even over aggressive.

The things you can do now. When on a walk and you see another dog. Stop, make your dog sit and begin watching for the signs of the aggression. Ears up and forward, body tense, leaning forward, those types of things. Correct then, before the barking/displays. You have to be firm. Use side jerks on the leash, just hard enough to make a change in the behavior. You do not need to jerk the entire dog, so the jerk is not punishment, just for diverting attention. Say NO! as you correct.

It is much better to prevent the aggression than to allow it and then try to correct. It is more difficult for some people to see the signs until they begin closely observing their dog. But, soon, the signs become obvious and you can start preventing faster. That makes training out that behavior easier. The initial several days to a week or so will seem like nothing is happening. That is fairly normal. The dog has to understand you(the dominant one) is not going to allow that behavior.

The other thing, initially, give no praise. You don't want the dog to learn, you require it to obey. Without reward. That may be the only area where that attitude is necessary, all the other things can be reward based. So you can carry the treats and pass them out whenever. Except for compliance to the no aggression rule.

There's good advice here, but since you said the aggression (which is pretty typical for JR's, as handymac described) scares you then you should enroll in an obedience class ASAP or have a private trainer come to your home. I find the obedience class work well since it gets your dog out among other dogs and it allows the obedience trainer to help you with the problem as it's happening.

There's nothing wrong with your dog that can't be corrected. You just need to learn how to train your dog......and not allow him to train you.

I have a terrier/Iggy mix. Before him we had schnauzers for decades. I love little JRTs but they, and really any terrier are an ilk unto their own with instincts. The know no fear.

Anyone who takes on a terrier breed should research them before the fact to see if they themselves have the temperament to hold those darlings in check. They get little dog syndrome if not. I got this one at the shelter, and before I committed, we spent days getting to know each other, taking walks and coming in contact with other dogs.

We have had some issues we have worked through successfully and I have a spouse who is bent on turning this pooch into a spoiled brat, lol. But this can be addressed and conquered and is so worth it. Fine little dogs for the right owners.