To enable monitoring or SPAN on a Cisco switch you can do the following:
monitor session session-id source interface/vlan
monitor session session-id desitination interface/vlan
Say you wanted to capture both transmit and receive traffic on int fa0/1 and send it to fa0/24 it would be:
monitor session 1 source interface fa0/1
monitor session 1 destination interface fa0/24
You would then connect your laptop to port fa 0/24 and run wireshark to capture any data sent/received on port fa 0/1.