I'm new to android app developing. I have a MySQL database online and I want do create an app that show some information that are on this database. I don't know Java.
Which is the best way to do this? Learning Java, creating a PHP webpage and show it on the app screen, ...?
"best" is highly subjective. I personally would recommend learning Java, since the SDKs and all the example code are written in it. Also, with Cordova, you are writing javascript anyway, so why not just make the leap to Java.
Here is a tutorial with PHP, MySQL and Java Android app connecting to it:
http://www.androidhive.info/2012/05/how-to-connect-android-with-php-mysql/