This might seem like a minuscule task, but for those that are very new to Juniper, this is for you.
A new Juniper device has just been installed and you need to only setup a default route.
Log into the device and do the following:
root@Junos# set routing-options static route 0.0.0.0/0 next-hop IP.OF.DEF.GATEWAY
Commit your changes and you should be good to go.