I'm about to install debian as my OS. It's not my first time on gnu/linux, just the first I'll be using it without dual booting windows.
Use stable if you hate updates and you wish everything stayed the same modulo security fixes.
Use testing if you want something stable.
Use unstable if you want cutting edge and can tolerate your distro breaking every other year.
Don't use experimental. If you're a noob, get Ubuntu instead.