You can enable/disable the serial console with either editing /boot/config.txt or raspi-config (which will edit /boot/config.txt for you)

Option 1. Enabling in /boot/config.txt

You can pop your SD card into a computer and edit config.txt with a text editor like SimpleText, WordPad or whatnot. You can also edit on a pi with sudo nano /boot/config.txt

At the bottom, last line, add enable_uart=1 on it's own line

Option 2. Enabling via Raspi-Config

Using a monitor and keyboard, log into the shell and run

sudo raspi-config

go down to Advanced Options

Hit enter and then go down to Serial

Select Yes

It should now be enabled

Hit return then select Finish

When it asks you to reboot, go to Yes and hit return

OK the serial console is now enabled!