I have a fiancé living in Cambodia (outside the U.S) and I plan to go there and get marry with her and bring her here to the United States and I wanted to prepared everything ahead of time, but I don't know where to start!

1. what is the first step I need to do?
2. what kind of forms do I need to prepared for?

Have you met her in person and gotten to know her family before now? Is this an Internet meeting and romance?

If you have physically met in the past two years, then you will file the I-129F petition with the USCIS. After I-129F is approved, she will apply for the K-1 visa at US consulate in her home country. When she enters the US, you must get married within 90 days and you file for her permanent residence.