Counting Cards?....Blackjack?....Help!?

okay, i know how to hi/lo count cards. But, i want to know if you have like negative 2. .....So? what does it mean? Should i hit Or something? And i watched 21 and they got up to 15. Is that with multidecks? When should i play? when i'm in the positive? i guess, since the low cards are out: meanig i get higher chances of getting 21 right? I've tried counting cards but, it really doesn't mean anything. What do i need to look for when i have postivie and negatives?