www.zagreb-touristinfo.hr/ Begin with the capital - Zagreb Offical Zagreb Tourist Board Pages
www.willerexpress.com/ Booking in English,Korean and Chinese.You can pay by credit card.
www.ask.com/travel+by+train Search For Travel By Train. Ask.com Provides Useful Answers.
How to travel by train
How I did it: I went back to Indiana (where I used to live. I now live in california) for a friend's wedding, and took the train home. I bought the tickets online and all. It was MUCH different from flying. I got quite bored and sleeping was difficult. They also lost my suitcase at one point, but I got it back later. It was worth it, though, because going through the mountains on the train was great.
Lessons & tips: Be careful with your bags, bring a neck pillow, and watch out for the food. Also, do your best to get the seat with the power outlet. You'll be going through areas with no cell phone signal, and that drains a phone battery really quickly.
Resources: The amtrak route maps were good.