For practical exams (such as ABRSM or Trinity College London exams), you will be required to perform your pieces from memory. This is because the examiner needs to be able to assess your technical ability and musicianship without the distraction of you reading the music.
There are a few exceptions to this rule. For example, if you have a disability that makes it difficult or impossible for you to memorize music, you may be allowed to use a music stand. However, you should always check with the exam board in advance to find out what their specific requirements are.