The formal word for hello is Kon'nichiwa (pronounced ko ni chi wa). If you're not familiar with Japanese, it may be easier to remember the slightly more informal hello Ohayo gozaimasu (pronounced Ohio go zi e mas). If all else fails the informal hello is pronounced Ohio (like the state).
If it's noontime, Konnichiwa (pronounced Kon-ee-chee-wa) is how you say hello.
If it's early morning, you say Ohaiyou Gozaimasu (pronounced O-hi-yo Go-za-eye-mass)
In the afternoon, you say Konbanwa (pronounced Kon-ban-wa).