If you want to start a brand new animation, select the 'Save & New' tab and click 'New Animation'. This will erase the current animation, so make sure you don't mind losing it.
If you need more help then please click here.
Before you start on the beautiful details of your animation, try drawing a simple outline. This way, you can get the overall composition looking great, and you can make sure everything's going to fit in the frame! Once you're filling in the details, you can start deleting your outline if you don't need it any more.