On the resistive TFT touch shield
Solder the jumper labeled Pin 3. Then you can use Digital 3 to control the backlight.
On the capacitive TFT touch shield
Solder the jumper labeled Pin 5. Then you can use Digital 5 to control the backlight.
Advanced users may want to get an interrupt on a pin (or even, just test a pin rather than do a full SPI query) when the touchscreen is pressed. You can do that by jumpering the #7 solder jumper labeled TS int. We didn't want it to connect to #2 or #3 since those are the Leonardo I2C pins. You can use pin change interrupts to get an interrupt callback on #7. Or, with a little blue wire, advanced users can connect a wire from the TS interrupt pad to any pin they choose. We find that querying/polling the chip is fast enough for most beginner Arduino projects!