if you want to rotoscope a selection use vector paint.
go to effects>paint>vector paint.
vector paint is a little bit finickey but it's the best tool for the job.
there is only 1 'undo' which is a bit stupid.
in the vector paint diaglogue box go to the first frame where you want to start rotoing.
In playback mode select 'current frame' and then create a keyframe for playback mode (click the stopwatch next to playback mode)
paint your mask. (paint tools are available in the comp window)
if you want your painted selection to be a matte select 'as matte' in the composite paint bit. I find its easier to leave this on 'over original' until I'm finished the roto. then switch it to 'as matte'or 'inverse matte' or whatever I want.
when you're done that first frame. go forward 1 frame (page down hot key) and paint another selection.
more here
http://www.creativecow.net/articles/...tte/index.html