What is the reason behind a human birth ???
First of all the reason behind your birth on this planet :
There is no specific reason for that. I should ask you why matter exists, why energy exists there will be no answers. Humans are also same as all other living creatures. The biggest aim of life is just to create offspring via reproduction. Why would there be any reason for you to exist.
Those who say that your aim in life is do good things, welfare etc. in life are all wrong. There is no such reason at all. All the other things and rules mankind and society has created are just artificial they are not your purpose or aim in life they are just there smooth running of society.
one more thing the term "reason " can't be defined. No matter how much a person thinks, how much science advances there will be a limit beyond which he can't know what it is or what is its purpose. for e.g if someone says that reason for your birth is to do good things, then you'll ask what are good things, why not to do bad things. Let's choose good things. Then the next question arises why good things and what are good things. Say good things are for welfare. Then you can further ask yourself what is welfare, in what terms profit,loss, money what?
So this question will keep on getting deeper and deeper and you will never be able to find the purpose of life if you keep on thinking and finding it constantly. It would be a lot better if someone never thinks about purpose of life (well that in fact is very difficult especially for me). There is no reason at all. All the world is just an illusion even many great religions like Hinduism and Buddhism also say so. They say you are here to achieve complete salvation i.e. when you realize that life is an illusion.
Now your 2nd question : what happens to soul after death ?
If you analyze what all i have just, I believe in soul and i think soul also dies after death. If you think of soul as transferring to other body and not remembering the previous life, it means that soul is also changed. Otherwise there is no meaning at all. Some say souls are never created, never die , But that thing i can easily prove wrong. You see how many souls are there at present and how many were there at the begining of life. From where does new souls are coming ? This question is open for interpretation. If your soul even exists and can't remember this life after death, then what is the damn use of thinking about this question.
3rd question : how to find that specific reason for your birth ?
As i said earlier there is no reason at all. so this question doesn't make sense. If you think everyone is here for a reason, then there is no reason at all to think what is my reason for birth. If others are not even bothered to find the reason still they will be unknowingly fulfilling the reason for birth. It will be automatically fulfilled if you have taken birth.
You should think a little deeper and read what i have said written once more. You will find out and all the answers about life