「知らない」[shi-ra-nai] can be used to mean "I don't know (and I don't care)," while 「わからない」 [wa-ka-ra-nai] means "do not understand," and is preferable for almost all situations. 「わかりません」[wa-ka-ri-ma-sen] is the polite form of the Tokyo dialect (standard news dialect) to explain "I dunno" to a superior, like a teacher or elder.