the router interfaces can be accessed by typing their name while in the configuration mode. Once that is done, the router prompt will end in (config-if) no matter what interface you are in. The most common interfaces are :
serial: s0, s1, s2, etc
ethernet: e1, e2, e3, etc
fast ethernet: f0/0, f0/1, f0/3, etc.
as an example, to get into the fast ethernet config mode from the user mode, type :
Router1> en
Router1# config t
Router1(config) int f0/0
Router1(config-if)
*** now, any commands you enter will be applied to the Fast Ethernet interface, numbered 0/0. For example, the router may have two Fast Ethernet cards, each with 4 ports. To access the first set of 4 ports you would use f0/0, f0/1, f0/2, and f0/3. To access the second card with it's 4 ports you would use f1/0, f1/1, f1/2, and f1/3.