Quite simple, run ifconfig and copy the following part from the interface(s) you use.
Also include uptime and what you're running :-)
Here's mine:
RX bytes:274521778650 (255.6 GiB) TX bytes:548799031176 (511.1 GiB)
up 11 days, 16:56
Running one Minecraft server and nothing more (I think I also download ~100GB world data)
I recommend vnstat, didn't have time to install it when I set up this server