Use your noodle！
Use your brain!
He gave you $10,000? Use your noodle！It’s not that easy!
Think twice 再三考虑
You'd better think twice if you’re gonna invest so much money in a project you know so little about.
Think long and hard 认真思考
I’ve been thinking long and hard about going abroad. I just can’t make a decision.
Think sth. over
I don’t think he’s the one for you. You need to think it over.
Have another think coming
If you think you can walk in here and just demand a raise, you've got another think coming.