Rotoscoping used to mean hand drawing or tracing over footage (see the Ring realm in Ralph Bakshi's animated Hobbit movie, just gorgeous), but anymore it mainly means using masks/paths to cut elements out of footage. It can be anything from cleaning up greenscreen stuff to completely isolating objects from unkeyable footage from scratch. It's a grind, and senior artists hate to deal with it, but it's a fine way for newbies to get their foot right in the door. Look for free jobs with second-rate clients to begin, build your skills, make a copy for your reel, and then start asking at better places. Basically take any job you can stand at first. It takes a good forty hours or so until you really get the hang of it, more or less. You'll probably end up tossing an hour of work here and there as you realize how you *should* have done it; just suck it up and move on.
Below I will outline some basic principles that should save you a little bit of learning-the-hard-way, which is how i did it.
The worst thing you could do is make one huge path on frame one, then adjust it on frame two, then adjust it on frame three. Not that you won't find people doing it that way, but it's really dumb.
-always work at at least 200%, or even much more. it's way easier and lots more accurate.
-i prefer to make a solid layer above the footage in question, turn off its eyeball, and create all my paths on that, and then set the footage to use it as an alpha matte. Otherwise you have to turn all your masks to "None" in order to work, and then set them all back to "Add" when you want to check your work, way too much work; this way you just turn the alpha matting on/off.
-use a contrasting color layer underneath to check your work: if you're trying to cut out a white background, check your work against a black solid, etc. in time you'll learn where the path needs to go to preserve the desired pixels without including edge/background crud.
-use multiple paths, not just one big one. wherever two curves cross (like an ear on the side of a head, or two legs crossing), give each a separate curve. If paths aren't needed in sections (like an ear that's hidden for part of the shot), move them off screen.
-your enemy is chatter, buzzy lines that are guaranteed to grab the eye's attention. too few curves and too many vertices and keyframes are the guaranteed way to make sure your roto buzzes and sizzles horribly.
-for really small details, page up/down to see if it looks like your path is wandering against the pixels. Your eyes are really good at noticing fine errors when there's motion (especially slow small jiggles), which is why doing this well is so valuable. I once did some eyebrows that were maybe all of eight pixels wide, shaped my paths way more precisely than the chunky pixels, but the final result wowed the supervisor nicely.
-first set keys at all the major change points, i.e. where the element changes direction or stops. then go back to the point in between each pair of points where the path is furthest off and adjust it, and continue working finer and finer until it's right on. most of the time only about a third or so of the frames actually have keyframes on them if you do it right. Some elements will be static while others move, another reason multiple paths help avoid chatter.
-Command T gives you the Transform box, which allows you to scale/rotate/move selected points, or the whole path (option click the path to select the whole thing)
-name your paths; hit Return to highlight the name, and again to accept the changes
-click the colored box next to the path name to change its color
-in Preferences, there's a "Preserve Constant Vertex Count..." option. this is a little complicated, but also not: if it's checked, and you add or delete a vertex (point), it will be added/deleted on ALL the keyframes. Adding a point means more points to adjust, which can grow very tedious, and also introduce chatter; deleting a point means all your other keys just got completely screwed up. if it's un-checked, and you change the number of vertices on non-adjacent keyframes, AE will totally screw up the interpolation between. try unchecking it, make a rectangular path, set a key, duplicate the key ten frames away, then add a point, and see what happens. not what you want. Rule: if it's unchecked, you can only add/delete points on adjacent keyframes, ie one frame apart.
-use the Arrow tool to select points, then the arrow keys to nudge them: right hand on the mouse, left hand across the keyboard on the arrow keys.
-if you are planning on filling the paths and making a graphic treatment instead of using the footage, don't try to make the same edge twice; just make one clean and make the other one overlap (underlap) it. An example would be where a shirt meets pants: do the bottom edge of the shirt, but just make the pants top loose and sloppy up under the shirt. Less work, and it'll look way better; two edges butting is nearly impossible to do well and there's no good reason to bother trying. With practice you'll learn to plan the sequence of paths/fills; work from big background bits to smaller foreground bits.
-You can also make multiple solids and then precomp them and use that as the alpha source. the advantage of this is you can move/animate individual layers, not just the path (like for a moving forearm), and gain the benefit of motion blur, or apply it manually (with Radial Blur and/or Directional Blur); moving bits don't have sharp edges. you might need to use the Paint tools here and there to take out background color interference in the blurry bits.
there's probably more, but by the time you absorb all this (hah.) you should be able to figure it out yourself. mainly just put in as much time as you can practicing and you'll stumble/bumble down the path just fine.