How people believe in heaven is through faith. Faith can be described as believing and hoping for the unusual and unbelievable stuff. Faith also believes when there is no proof. I guess that some people have a lot of faith and that is why they find it easy to believe in heaven.
Start reading the bible, if you believe in god you believe in heaven. Sometimes for me its hard to believe in heaven. You should look at those people who talk to the dead. That made me believe in heaven even more because the dead is alive up in heaven. Never stop believing ~ hoped this helped :)