"I will need to change my opinion/ideas." Usually it is used when speaking to someone else. "You haven't revised, you go to parties every night, and you hope you will pass your exams with distinction......you've got another think coming!"

It literally means: "Something else is coming this way." It has a number of uses. You'll have to post the example where you saw it.

To say that someone has "got another thing coming" means that you're about to experience something unexpected or unpleasant. This phrase is usually used as a threat, so it's not a something that you would use while talking about yourself.

This expression has evolved from its original form. The original expression is: If that's what you think, you've got another THINK coming. In answer to your question, it's an expression that opposes a stated opinion. In a sentence: If you think you're getting into Harvard University with poor grades, you've got another thing coming.

People often say " You've got another thing coming if you think that..........." which means that your opinion is wrong.