Unfortunately on a "dumb" switch (as opposed to a managed switch) it isn't that simple to see all traffic or traffic to/from a specific port other than the one your sniffer is plugged into.
On a managed switch you put the port your sniffer is plugged into into monitor mode (or span mode in Cisco money) which makes that port promiscuous, then you can see *all traffic.
As it is with a dumb switch, if you run wireshark on your desktop you will only see traffic to and from that machine + any broadcast packets.
If you have a machine with 2 ethernet interfaces you can build an ethernet tap either by making it a bridge and sticking it between whatever is in port 8 and the port itself or by doing another trick with some spare rj45 modules see
http://www.enigmacurry.com/category/diy/ for a reasonable guide on how to do this.
Finally you can use tricks such as ARP poisoning or MAC flooding but both of these are really only suitable for espionage rather than diagnostics as you change the behaviour of the network by utilising them.
* Well not quite all traffic