I'd actually disagree with most here and advise you to go to host only mode. This means go to File > Preferences > Network and click on the add adapter button. This creates a virtual network interface on your computer.
At that point you can go to your vulnerable VM > settings > network > Host-only adapter. This means only your main PC which is running the VM or other virtual machines can interact with the VM. You still get separate IP addresses etc, but you should really never have deliberately vulnerable machines on your live network. It will be perfectly suitable for pen testing, and is infact the recommended setting for deliberately vulnerable VM's such as metasploitable.
Find your IP address the way you would for that style of machine, typically typing ipconfig in cmd for windows or ifconfig in terminal for a linux based system.
This protects you accidentally exposing it externally.
Edit: Dammit I didn't see this had been raised from ages ago for some reason.