Normally you would just set the network to 'NAT' configuration in VirtualBox.
To set the IP address, log in as the user account you created during setup and run:
Enter your account's password when prompted.
sudo nano /etc/network/interfaces
Look near the bottom of that file for "iface eth0 inet auto"
Change it to look something like this, but change IP addresses etc to suit your setup.
To save and exit: Ctrl + O, Enter, Ctrl + X
iface eth0 inet static
To restart networking:
sudo /etc/init.d/networking restart