Three very good questions. Let's take them in order.
How do teachers get tenure? Here is a definition of tenure I found on my county's website. Florida public school systems are required to collectively bargain with employees on all matters affecting their terms and conditions of employment. Teachers in public schools, after completing a probationary period, are granted tenure by law and may not be dismissed without first going through a year-long rehabilitation period and then only after a public hearing in which charges of incompetence or misconduct must be proven. Private schools are free to deal with personnel matters in a discretionary manner, unencumbered by requirements imposed on public schools. http://www.browardschools.com/info/education.htm
In other words, tenure is tied to the teachers' contract. It is designed to protect public school teachers from disgruntled administrators, students, and parents.
How long does it take? Each state is different, but I believe the average is 3 years of successful teaching. If you want to see how long it takes for teachers in your state, check out http://certificationmap.com/states/.
How difficult is it to obtain? Not hard, well it wasn't for me. I was surrounded by good teachers, students, and parents. I didn't get a great administrator until year 5. For the record she was fantastic and helped me to become a better educator during the 10 years she was my principal.
Glad to hear more people interested and asking questions about education. Keep them coming.