QoS is something you should set up when you have a bunch of users and VoIP. You really want the VoIP to run in its own VLAN separate from the rest of the network traffic.
You should do QoS marking as close to the source as possible (at the switch port, if possible). Even better, the phones may already mark the VoIP traffic as EF, but that depends on the phone. The rest of your traffic can be marked as BE, or you can get into multiple classes if you have a need for that.
Once the traffic is marked, you need to set up proper queues in the router for the various traffic types based on the markings. VoIP should probably have a priority queue which you need to size carefully. There are various other techniques such as shaping, RED policing, etc. QoS is a topic far too broad to properly be discussed here.
Remember that, unless you pay for it, your ISP won't honor QoS markings, and any other ASes through which your traffic travels to your VoIP provider will not honor any QoS markings either. The goal for QoS in your network is to make sure that VoIP gets first priority exiting your network.