How to monitor internet usage over a network?

Hi,

I'm a newbie of using WireShark, just told by our network provider to use it in order to check internal usage of bandwidth as they are saying, it's perfectly fine but we are experiencing very slow internet connection as we have many tenants who are using same internet. It was working perfectly fine about 2 weeks ago but there's something ambiguous activities going on, so, I would like to check which IP or who is using infact abusing the internet.

Early reply should be really appreciated from the seniors or moderator. Thanks, Nadeem