well, first off i think that you are long overdue for a visit to your prelaw advisor's office... he/she will answer all of these basic questions w/in the first 5 seconds of you walking in the door. in short, do your homework!
that said.. here is a quick answer to your question:
there is no such thing as "prelaw" as a major... it only states your intention to go on to law school. as such, there are no pre-requisites to get into law school (other than the 4year degree). therefore, you could continue with your nursing school program and then go on to lawschool w/out missing a beat.
law school is 3 years of full time study or 4 years of part time study.
and of course you apply to law school... same as undergrad! this seems to work better than everyone just showing up on the doorstep of their LS of choice
after law school you must still pass "the bar" in order to be a licensed attorney. this exam is offered twice a year (july and february) in each state. with some exceptions (see: reciprocity) you must pass the bar in each state in which you wish to practice law. the bar tests the minimum competency that is required of an individual to practice law in that state.
when you say I, II and III lawyer, i am assuming that you are referring to the years of study of a law school student (1L, 2L and 3L). for example, a 1L is the equivalent of a freshman in undergrad.
for more info go to www.lsac.org