AE is not the strongest tool for rotoscoping. If you have Pro, you can make a mask and then interpolate the mask from keyframe to keyframe, but AE is not the best at it (at some point, Adobe is supposed to upgrade it’s roto features, but apparently that didn’t happen for 7.0).
You’re better off getting a plug-in like Roto by Silhouette FX (www.silhouettefx.com), or even going with Combustion 4, which does roto really well.