to think up or decide on an opinion. (Note the variations in the examples.) I don't know enough about the issue to form an opinion.Don't tell me how to think! I can form my own opinion.I don't form opinions without careful consideration.
Make up one's mind or decide what one thinks about something. For example, I need more facts before I can form an opinion about this issue, or Don't tell me your views; I want to form my own opinion.