But what if you can't boot into X for whatever reason, or if for some reason you can't start nm-applet or equivalent? If you leave network setup to NetworkManager, you have no network now. This also means no way to google for help. :-)
You can find many guides telling you to just configure the network yourself. If it's a wired network this is quite easy (with DHCP just dhclient eth0 would often just do the right thing). But for wireless networks this gets more complicated. So you might be advised to use iwconfig (for open or WEP networks), or wpa_supplicant (for all).
But if you already have the network configured in NetworkManager (and the NetworkManager service is running, or you manage to start it), I suggest you just use that. There's a command line utility to interact with the NetworkManager, called nmcli.
Here are the basic usecases you might need:
nmcli con listlists all the configured networks NetworkManager knows about; notice the columns NAME and UUID.
To bring up a connection use either:
nmcli con up id NAMEyes, I also find it a bit confusing - but you really are supposed to put a value from the name column after the id :-), e.g. nmcli con up id MyNetwork
nmcli con up uuid UUIDyou normally don't need to use this
To bring a connection down use down instead of up.
For more details see man nmcli, and nmcli --help.