Jenny, "hi" is the same thing as expressed in Tagalog ('hay'). "kumusta" is the most formal way to equal this word. 'hoy' or 'uy' is sometimes used to express a surprise. "hoy / uy, kumusta na? tagal na nati'ng di nagkita a." "hey, how are you? long time no see." (be warned of using "hoy" as it can be sometimes informal and offensive in addressing someone). :)

Hoy is rather rude if by itself. When speaking to someone you know, the intonation matters a lot and the manner it is delivered. Usually, when speaking to someone you know, you say it in a phase which is, hoy, kamusta? but it is pronounce as uy, kamusta? To someone you don't know or is older, you say " Magandang araw po, mawalang galang na po " which means good day, please excuse my intrusiveness.

we say "kumusta?" for hi hoy is "hey!" in English.:)