Terminal open: click the Applications menu on the top of the screen (left side), there open Terminal from the System tools menu (or equivalent..I've not used english version for a while). It's the icon that looks like a black tv screen.
ifconfig needs to be run with root privileges, so in Ubuntu you use sudo to achieve that (because root account itself is, by default, locked for security reasons and should remain so); in addition ifconfig is under /sbin/ directory if I'm right, and that directory is not added to the regular user's $PATH environment variable by default (again, if I'm right)..so you need to type in the full path to the executable for your shell to find it, the plain executable name might not work (usually it's all you need). So it works like this
then it asks for your user password (except on the live-cd, where it won't ask any), the one with which you logged in. After typing it (nothing is displayed on the screen, but it is working nevertheless - just type carefully), ifconfig is run with root privileges and you should see the devices that are set up (at least 'lo', usually 'eth0' and maybe 'eth1' or 'wlan0').
You can try to fetch everything needed for your ethernet card (which is usually called 'eth0') via DHCP in Ubuntu like this:
sudo /sbin/dhclient eth0
some distributions use 'dhcpcd' instead of 'dhclient', but the above should work for you. See what it does. If it does not create any output (after the command you just get a "new command line"), it worked - if you get output, something went wrong.