if ur canvas have some parent component then there u can provide
horizontalAlign="center"
like
Code:
<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" pageTitle="Login"
xmlns="*" width="100%" height="100%" horizontalAlign="center" >
<mx:Canvas label="Dynamic and interactive websites with PHP" width="100%" height="100%" >
<mx:HBox width="100%" height="100%" horizontalAlign="center">
<mx:TextArea height="100%" width="90%" />
</mx:HBox>
</mx:Canvas>
</mx:Application>
or one more thing u can do, u can provide padding (either left or right).