やあ、こんにちは。 [yaa-konnichiwa] While most of the time こんにちは [konnichiwa] will do for saying "Hi" or "Hello," if you are on casual terms with someone such as a close friend or a youngster you can use expressions such as 「おい」 or 「おっす」 [oi!] or [ohh.s.] although these come off somewhat brusquely if you are speaking to a superior, elder, or new friend, but are commonly used among men who are on the same level in the social-hierarchy and know each other already. When answering a phone call, people typically say 「もしもし」 [moshi-moshi] which translates into "if if" but is more akin to the idea "Hi, anybody there?" There are many casual ways of saying "hi" and what is in vogue is often changing. If you stick with 「こんにちは」 you can never go wrong, unless the situation is extremely casual in which case a smile and a nod or slight bow will also do.