In IDPA, you don't count up points on a target. Rather, you try to hit the "zero down" zones. Hits outside of those (or complete misses) count against you. As in golf, the guy with the lowest score wins.
You can download the entire rulebook here: http://www.idpa.com/rule_info.htm
You should be good enough that you know the basics of handgun safety, and can safely draw and reholster a gun. If you don't worry about your score, you can learn a lot and improve at your own pace.
Most IDPA clubs don't charge a lot. Members of our range pay $12 to shoot a monthly match, non-members pay $17. You also need to join the national IDPA, which costs $35 a year.
As someone mentioned, IDPA stages are limited to a maximum of 18 rounds required. Our monthly matches are 6 stages, so a hundred rounds give or take works. You could check with clubs in your area and ask how many stages they generally shoot.