I want to make a desktop app obiously im a begginer so
if i built a gui in mac using python and tkinter that program will work on windows?
Also is tkinter the best framework?
Yes, but tkinter applications will look different on different platforms. This means that you may get different buttons on Windows 7 vs MacOS X -- Tkinter is just using whatever the OS gives it.
As for it being the best framework, I couldn't say if it is the best or not, but it is pretty simple and works well. I normally use tkinter.
Python tkinter Mac and windows