If you are a beginner, or your internetworking experience is limited, your first step is to get a solid grip on the basics of both internetworking and Cisco routers. This effort will involve both classroom training and self-study.
Through its training partners, Cisco offers many hands-on training classes. You should attend as many of these classes as your time and resources allow, but of particular importance are:
Introduction to Cisco Router Configuration (ICRC)
Advanced Cisco Router Configuration (ACRC)
Cisco LAN Switch Configuration (CLSC)
Cisco Internetwork Troubleshooting
Take full advantage of every class you attend. Ask questions of the instructor and discuss the class topics with your fellow students. Most important, ...