If you're just getting started and you're not trying to make something
super slick, I'd suggest Tkinter. It's easy to learn and use, you can
bang stuff together with it pretty fast, it's included with various
Python distributions so you avoid download/installation hassles, and
it's pretty portable across various desktop OS's (not mobile for some
reason). The downside is that you get industrial-looking UI's that
implement typical GUI functionality but don't have ultra precise control
or carefully crafted widgets like some of the other toolkits do.
Kivy (kivy.org) also seems worth looking at if you're trying to be
cross-platform. It runs on both desktop and mobile. Although, one of
my mobile-using buddies tells me that mobile apps are now passé and
these days people just write web apps for mobile.