The first step after installing Raspbian on your new Raspberry Pi is to Expand the Filesystem! The image only allocated 4GB on the SD card, even if the card is larger than that.
Note that the newer versions of Raspbian will need an SD card larger than 4GB. I recommend at least 8GB to start.
The version I run is:
pi@raspberrypi:~ $ uname -a
Linux raspberrypi 4.1.18-v7+ #846 SMP Thu Feb 25 14:22:53 GMT 2016 armv7l GNU/Linux
After successfully booting my new Raspberry Pi 3 with a 16GB SD card and interacting with it in the GUI, I was surprised I could not reboot into the GUI. After connecting via ssh and looking at the log files, I noticed that the window manager could not write a file, receiving an out of space error.
I looked at my 16GB SD card and noticed that only 4GB was allocated!
To correct, and the first thing to do after booting up the latest version of raspbian, is to expand the filesystem. To do this, run
raspi-config (or select Raspberry Pi Configuration from the Preferences menu) and choose “Expand Filesystem.”