How do I know a tornado is coming and what do I need?

I live in Revere Massachusetts, close to the water. ALL DAY the weather has been cold, hot, cold, hot, cold, hot. Its cloudly and kinda dark out. Dark clouds are scattered everywhere, but kinda oddly. Its warmishh right now, it was raining alot outside last night.. How can I know a tornado is coming ?? What do I need ?? Im only 11 & really scared because Springfeild recently had one. Please answer !