Once you arrive, pick up a visitor's guide (separate editions for Phnom Penh, Siem Reap, and Sihanoukville), as well as any of the various pocket guides, the best of these being Cambodia Pocket Guide (www.cambodiapocketguide.com), widely available free at airports, hotels, and restaurants.
Andy Brouwer (www.andybrouwer.blogspot.comorwww.andybrouwer.co.uk), a longtime traveler to Cambodia, has dedicated a good part of his life to informing people about the country where he lives. Tales of Asia (www.talesofasia.com) is an excellent source of information, with travelers' stories, road reports, and up-to-date travel information.
The Ministry of Tourism (www.tourismcambodia.org) has some information on its website. Tourism Cambodia (www.tourismcambodia.com) has more detailed descriptions of top attractions. You can also visit the Tourist Information Center in Phnom Penh at 262 Monivong Boulevard, Khan Daun Penh.