There are various routes depending on your experience. If you have previous coaching experience or played at a high level (college or above), you could apply for a graduate assistantship position. The NCAA website has an employment page that lists open positions. Without these experiences, you will most likely have to begin at the high school level, either while working as a teach or perhaps as a volunteer coach.

I would also suggest attending coaching conferences such as the American Football Coaches of America (AFCA)convention. Information is available on their website www.afca.com. These conventions are great for learning about the intricacies of the game as well as making contacts through the profession.   

The timeline for progressing to coach a college team are again dependent on your experiences. If you are able to attain a graduate assistantship, you will typically be in that position for 1-2 years and then will be able to get a job as an assistant coach (typically a position coach), a few more years and you may be able to advance to a coordinator position. And from there you can begin to explore opportunities as a head coach. If you are starting at the high school level, it may take a few years of work there before being considered for a graduate assistant position.

Let me know if you have any further questions.