Preparing for an exam, especially if it is your first grade, can be daunting and it is hard to know what to expect! It is important to remember that the examiner is there to give useful information about your playing and to look at things that could be improved as well as commenting on aspects of your playing that are successful and positive. Performing beforehand on lots of different pianos will help you become accustomed to the feel of different upright and grand pianos as they all have a very different feel. It is also a good idea to perform to as many friends and family as possible. This will help nerves as it can feel very different performing in an exam situation compared with the relaxed nature of a piano lesson. Above all, it is important to enjoy your exam and see it as a positive performance opportunity! Regular practise will help you become comfortable when playing your pieces and scales and your teacher will practice the aural tests and sight reading with you on a regular basis.
For more information on the preparation involved in taking your graded exams, follow this link: http://www.abrsm.org/resources/theseMusicExams0607.pdf