When you first get out your flute and warm up, you will be flat. As your flute gets warm from your breath, you will become sharp. In a concert, everyone's pitch will get sharp because of the heat in the room. You need to learn how to correct your pitch as you go along. If your pitch is one way or the other to start out, read below.
If your pitch is too sharp blow lighter. You can do this by bringing your jaw back, or opening your teeth more. Another problem could be that your Jaw is too far forward. To fix this, you can either bring your jaw back, or open your teeth more.
If your pitch it too flat try blowing harder. You can do this by bringing your jaw forward and opening your teeth up. Another thing that might help is to sit up straight. Make sure that you can support your sound from your lungs.