Invite some friends or family to attend a “mock” performance that you will give. They do not need to be musical at all – just ask them to write down some comments as you play (you could use the performance feedback forms for this).You can use these comments (good or bad) to help make your next performance even better.
As the performer you need to prepare yourself exactly as you would for your performance. This means you need to warm up properly, dress appropriately and play as if in a performance (ie play all your pieces from start to finish without stopping).
Do this a few times before your actual performance and you will be well prepared and more confident.