I have already appealed to the selected objects, and found a way how to put them in a MsgBox. But I need to output was in the form
GUIs for QGIS plugins should be written in QT since QGIS itself is written in QT too and it is cross-platform.
Get the excellent QT Designer: http://qt-project.org/doc/qt-4.8/designer-manual.html. It's cross-platform too.
Are you aware of the built-in functionality of "Edit UI" forms that can be specified in Layer Properties - General? Depending on your use case this could reduce the programming work you have to do yourself. See http://woostuff.wordpress.com/2011/09/05/qgis-tips-custom-feature-forms-with-python-logic/ and http://woostuff.wordpress.com/2012/03/30/custom-qgis-feature-forms-value-binding/.