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. -- https://mail.python.org/mailman/listinfo/python-list
