There are two networking modes that you need to choose from:
Bridged mode, where the virtual machine is effectively plugged into your router by a wire alongside the connection between the host computer and the router. The guest gets its IP address etc from the router, just like having an extra physical machine, and has access to and is accessable from, the outside world in the same way as a physical machine connected to your router.
The other networking mode that you could use is Host-only. In this case the virtual machine and the (virtual) second network socket on the host are both connected to a router that is built into Virtualbox, so it has its own DHCP server etc. This virtual router has no connection to the outside world so the virtual limited is only able to see and be seen by the outside world via the host machine.
Once you have chosen the network mode you want, you need to be able to find the address of the virtual machine (using
ifconfig on the virtual machine will work) and away you go.