If "native" command is NOT supported then configure as such:

hostname ROUTER-ON-A-STICK
!
interface Fastethernet0/0
 description Dot1q Trunk to Switch-2950
 ip address 10.1.1.1 255.255.255.0
 no shutdown
 speed 100
 full-duplex
!
interface Fastethernet0/0.10
 encapsulation dot1q 10
 ip address 10.1.10.1 255.255.255.0
!
interface Fastethernet0/0.20
 encapsulation dot1q 20
 ip address 10.1.20.1 255.255.255.0

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 
If "native" command IS supported then configure as such:

hostname ROUTER-ON-A-STICK
!
interface Fastethernet0/0
 description Dot1q Trunk to Switch-2950
 no ip address
 no shutdown
 speed 100
 full-duplex
!
interface Fastethernet0/0.1
 encapsulation dot1q 1 native
 ip address 10.1.1.1 255.255.255.0
!
interface Fastethernet0/0.10
 encapsulation dot1q 10
 ip address 10.1.10.1 255.255.255.0
!
interface Fastethernet0/0.20
 encapsulation dot1q 20
 ip address 10.1.20.1 255.255.255.0

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
Interface Range command: (interfaces 1 - 8)

config t
int range fa0/1 - 8
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
Configuring the Duplex on the interface:

Switch:

config t
int fa0/22
speed 100
duplex full

Router:

int fa0/0
full-duplex
speed 100