Use the pronoun вђс (вІ) and the form здрђвствуйте in addressing people of greater age and/or status, e.g. your teacher; use the pronoun тебї (тІ) and the form здрђвствуй (without -те) in addressing fellow students.
A good rule of thumb is to use вІ until you're advised otherwise. The exception to this rule is addressing small children and teenagers, where вІ is not appropriate in modern Russian.