Heck, when I started.. I was doing Neck Ups and working on just rolling over using my core rather than momentum. Now, I do tons of ab stuff. My trainer has me doing 100 ball crunchs a day with 3 lbs weight held at neck level - up to 500 a week. The last 2 times, I was actually able to get all 100 in one set.
With all that said, I still can not really do the plank. I try periodically and sometimes I can get 5-10secs and other times, I can't even get into correct form. I started working towards the plank, by holding myself in knee push up position, then full pushup position.
It really is baby steps to strength. Start where you are and do what you can. Next thing you know, you'll go to move in a certain way and you'll be using those abs you thought you didn't have.
Also, those legs/hip flexers, will get stronger as you work them as well. Try the heel slide and straight leg raises in:
http://www.fairview.org/healthlibrar...liopso_art.htm
or the stability ball hip flexion:
http://www.fitnessthatfits.net/exercises.html
My newest ball exercise is a quad pulse - similar to the leg press on this page but putting feet much closer in and only moving about 2 inches rather than full range of motion
http://exercise.about.com/library/blbeginnerball.htm - I'm working towards squats and lunges but my hips and quads are not strong enough for my current weight.