spend a few day in the capital city, Phnom Penh and you should heading to Siem Reap for Angkor Wat.
you could travel by bus from Phnom Penh to Siem reap at a low fares. boat is also accessable but there might be some problem and risk associated. I have used the bus before from Siem Reap to Phnom Penh 3 years ago. you could spend two or three days here!!