To install Qt5 package needed to develop do :
sudo apt install qt5-default
Create a new folder to contain qthello project files.
mkdir qthello
cd qthello
Edit a new file hello.cpp with nano (or other editor).
nano hello.cpp
Create qthello.pro file with qmake.
qmake -project
Edit qthello.pro add QT += widgets.
Run qmake and make.
qmake
make
Execute qthello.
./qthello
Done.