Etherchannel or "trunking" on a Cisco 3560G

Pretty simple actually, select the ports you would like to trunk.

conf t
int range g0/1 - 2
channel-group 1 mode desired
switchport trunk encapsulation dot1q
switchport mode trunk

You can check if the ports are trunked by running...

sh int po1

sh eth 1 sum or det