How can connect my android app to car system for example to get "Level fuel" information or any others info!
All modern cars have CPU which measures and controls few parameters of its engine. These CPU(s) will have an interface socket (looks like SCART connector) with which issues are diagnosed at service center. There is a special hardware which goes into this socket and can transmit engine parameters through Bluetooth. Google Play Store has an app called Torque which when installed in an Android phone, can receive such parameters and displays on the screen.
I am not too sure whether the OBD device can detect fuel level, but it does sense a lot of other engine related parameters like HP torque etc.
You would like to try Fuelio which advertise to give fuel and other info that cost money. I haven't tested it, however.