To do well on the TOEFL iBT, you should try to develop an attitude of relaxed confidence.
Of course, this doesn't arise by itself. It grows within you as you take specific and positive actions to prepare yourself in all ways – academically, mentally, physically, and psychologically. You are the one in charge of your attitude! Do whatever you need to do to help yourself, just as you would advise your best friend or younger brother or sister.
If you have been a good student in the past, draw upon these positive memories to boost your self-confidence. Similarly, if you have not done well until now, remind yourself that at any moment, your life can change for the better, once you decide to make it so. Read through and implement the extensive test preparation and test anxiety reduction techniques included on this website to help you achieve your goals.
As the famous saying goes,
Whether you think you can or think you can't, you're usually right.
Make sure you prove the best about yourself, to yourself, today and always.