For safety you may use another junk computer
If it's possible, do this.
I would like to clarify that Virtual Machine will still have vulnerabilities and if a malware writer discovers this he may exploit it. Meaning, you may one day end up with a sample which will escape the Virtual Machine. Without that being said, make sure your Virtual Machine software is up-to-date in this case (since the company who develop the VM software may then patch it up, however they would first need to be aware of the vulnerability which may take some time unless there is a big headline about it or they do malware research).
I really recommend having a great Firewall installed on your system before attempting to do any testing in a Virtual Machine.
Please make sure to keep the connection settings set to NAT.
Please be aware that the sample you are testing may enumerate all the processes as it starts, and check to see if there is a process with a particular name (basically a target search for the processes the Virtual Machine uses, same applies for Sandbox). They can do this to try to detect if they are being virtualized/sandboxed.
I also want to clarify that there are also other ways for malware to test if it's being virtualized. So be careful of samples which may not act very suspicious, but may actually be malicious software.
I also want to note here that I recommend using VMWare Workstation. I personally feel that it is more secure than VirtualBox. Of course many people use VirtualBox, you can if you'd like; however, I do recommend VMWare Workstation over it. I also feel that VMWare Workstation may perform better (generally speaking) based on my experience with both VirtualBox and VMWare Workstation.
Please make sure to keep a backup of your system just-in-case of any problems (your personal documents etc).
In your Virtual Machine, you can create a snapshot when the Virtual Machine is first created. This will allow you to restore back the state after the Virtual Machine is infected without having to reinstall the whole Operating System on the VM. This can be tiresome and snapshots are just a great feature for anyone using a Virtual Machine to revert back incase of any issues in a matter of a few seconds - a few minutes.
As well as this, don't mess around unless you really know what you are doing.
Cheers.