"You need to think" is very general. It could mean you need to think about anything at any moment. "You need to be thinking" is immediate. It means you should be thinking about something right now. Here is an example:
"You cannot keep wasting time. You need to think about your future." This means that at some point in your life, you need to plan for what's ahead.
"You cannot keep wasting time. You need to be thinking about your future." This means you need to start thinking about your future immediately (maybe because you've wasted enough time).
I hope this helps. Please let me know if it doesn't.