Need help to monitor internet usage in my LAN

You can use a managed switch to capture the traffic you just have to put it in the correct location. You would want this to replace the switch that is just before your router. You would then capture traffic on the line going to the router.

This is the simplest method but the output from wireshark sometimes too detailed so you end up processing huge amounts of data. I would still recommend you go this route and spend the time to learn wireshark well.

Another method commonly used is to place a linux based machine between the top switch and the router. You could then see the traffic like wireshark or you could use something like a transparent squid proxy. This tends to be the next step most people have to take after they capture data. You will find out who is doing stuff but you have no way to stop them. You end up putting in a device like this to act as a form of firewall to filter traffic.

A last option that is used for longer term reporting is to run netflow. This again can be captured by a inline machine or you can run it on some routers, ones that run dd-wrt can generate a form of netflow. This is a simplified version of the capture data you can get with a sniffer.

Still when it comes down to is your problem is a people problem and not technical. If these are employees maybe a reminder that if you catch them doing stuff that is not allowed they are out the door.