Your config would look something like this:

int f0/0

no ip address

int fa0/0.1

encapsulation dot1q 1 native

ip address 192.168.1.1 255.255.255.0

int fa0/0.10

encapsulation dot1q 10

ip address 192.168.10.1 255.255.255.0

int fa0/0.20

encaps dot1q 20

ip address 192.168.20.1 255.255.255.0

If you have a switch, you'll need to add the ports that you want as access ports, and then you'll trunk the port that leads to your router.

For instance, if port 1 connects to the router and port 2 is on vlan 10, and port 3 is on vlan 20:

int fa0/1

switchport mode trunk

switch trunk allowed add vlan 1 , 10 , 20

int fa0/2

switch mode access

switc access vlan 10

int fa0/3

switch mode access

switch access vlan 20

The syntax may not be right, but that should work. :-)

John