Local SPAN port on a 2960G

From global config

monitor session session_number source {interface interface-id | vlan vlan-id} [, | -] [both | rx | tx]

ex: switch(config)#monitor session 1 source vlan 1 , 2 both
NOTE: notice the spacing when specifying more than one VLAN

monitor session session_number destination {interface interface-id [, | -] [encapsulation {dot1q | replicate}]}

ex: switch(config)#monitor session 1 destination interface g0/12

Documentation from Cisco