The simple way to set up networking for a virtual machine running inside VirtualBox is to set up a host-only adapter and a NAT adapter. Let them both use DHCP (VirtualBox has a built-in DHCP server). Usually, this just works.
Only use a bridged adapter if you need a more complex networking setup. The most common reason is to have the virtual machine completely reachable from the outside. If you only need a few ports to be reachable from the outside, do it through NAT (see the VirtualBox manual).
If you use bridged mode, be prepared to have to learn more about networking. You can still use DHCP if there is a DHCP server on the network and it's willing to give you another IP address. You'll need to talk to your network administrator about this — some places only give out IPs to known machines and only one per machine (per MAC address). If you want to use a static IP, you'll also have to talk to your network administrator, so that the IP you pick is routed to your machine and doesn't conflict with another machine's. If you are your own network administrator, need bridged mode and don't understand what to do, you can get help here if you describe your network setup.