Pro tip: Another slang modification is "Hoj!", but use it only with your drinking buddies or close friends. Also, while not grammatically correct - you can use the plural form "Ahojte!" when greeting many friends. And "Ahojky" or "Ahojík" is another not grammatically correct word, however, it is a cute way of ahoj, but use those ones only for small kids or if you are a woman, it sounds weird from a man.
Warning for others: čest is a short for Čest práci, the greeting among communists. It may be understood as a joke ... or not. I can't confirm we use it too often...
Čau ... at least don't use it for people you don't call by their first name. It is more subtle, but at least this.