I don't mean to pick at you. I find it highly irresponsible of your mother to allow you to walk a dog that a) doesn't know how to walk politely and b) has leash aggression issues. Also it is illegal in most States for anyone under the age of 16 to be in charge of a dog in public. Walking your dog has already almost had you hit by a car, I think it is time your Mum starts to take some responsibility for a dog she bought for a child who was too young to really understand what was required to properly raise a behaviourally healthy dog.
As Beloz stated your dog has leash aggression. Fairly common especially in dogs who didn't get socialisation and whose leads have been mishandled.
At 13 I am very sorry but you are much to young to solve this on your own. You need to speak to your Mother about getting some professional help before you or somebody else gets hurt, if your dog attacks another dog during one of his outbursts on lead and you or another person try to intervene you or they could be bitten. Even worse what if next time he pulls you across the road you DO get hit by a car.
It is not an easy fix and will require months of hard work on desensitizing and counter conditioning.
Absolutely do not get another dog right now.
Again please do not take this as an attack on you, the fact you have come looking for help and you are trying to do the right thing by the dog tell me you are a responsible young woman.