Can I use native Qt5 C++ library to create for Ubuntu phone. Is there any guide or wiki for it. How can I test my app on real device(Galaxy Nexus). Where I can get recommendations for Ubuntu App Designing, documentation of Ubuntu SDK QML components.
The Ubuntu SDK is based on Qt5 respectively QtQuick 2.0 and QML. So you should be able to use all Qt5 libraries. Further information you could find on the Ubuntu SDK, the Qt5, the QtQuick and the QML Documentation pages.
To test your application on a real device you have to wait until the release of the first version of the Ubuntu Phone OS which will be released next week (February 21st). The 3rd generation Galaxy Nexus will support the Ubuntu Phone OS but there will be other devices. For further details have a look at the Ubuntu Wiki and the Ubuntu Operators page.