You can try a tool that comes with AE called Rotobrush. You will need to take some training to use it properly. The best option for finding training quickly is the Search Help field at the top right corner of AE.
Another option is to roto by hand. Most of the time that is more efficient if you use more than one mask. Sometimes it helps to "motion stabilize" the shot first. Here is a qui tutorial I did a while ago on hand roto.
Be forewarned: roto is extremely labor-intensive/time-consuming.
Before you start clicking mask points, make sure you figure out EXACTLY what you need to cut out, exactly when, and why. There are a variety of techniques for extracting shapes/edges/etc., and the right one may depend on what you're actually doing with the footage in question. You also don't want to spend 3 days roto-ing an entire person, only to realize you really only needed to mask out their arm.