How do I get started with building software like billing software or invoice maker. I made them with tkinter but I want them in attractive modern GUI . I don't want them to be web based. if its not possible with python I can learn other way too. but I don't know how.
I had a very similar issues regarding the aesthetic of a GUI. Perhaps have a look at a good few examples of GUI framework programs here:
https://www.digitaldesignjournal.com/python-gui-framework/
Although they are Tkinter based and probably the same structure overall. It might be a good option for your needs, as I understand.
Please also note that the limitations of GUI aesthetic is also limited in plain tkinter as far as I am concerned.
You can also have a look at tkinter.ttk as follows:
https://www.pythontutorial.net/tkinter/ttk-style/
I have played around a lot with the ttk attributes and it might be a good option to investigate. I find that python is a much easier language to adapt to rather than learn another language solemnly for better GUI aesthetic. (Although important)
I trust that this will help.