How to open a second window in QGIS plugin from a button inside the main window

If I already created a qgis plugin, and i need to create a button inside that plugin that opens a new window, how is that possible? Qt creator won't add a slot unless there are a header file. I looked inside the plugin directory and didn't find the .h file, so I compiled it using uic, but it still can't find the .h file it says : no document matching ui_test.h could be found, rebuilding the project might help.