There are two ways to go about this. The first is to simply add a solid above the video, change the blend mode of the solid to something like screen so you can see through it, then draw two masks that follow the outline of the fingers. Both should be open paths. Now simply apply Stroke to the mask path and have it paint on transparent and fix the blend mode.
The second option is a little more complicated. I'd start by placing markers on the timeline or on the clip where you want the path to start to be drawn and where you want it to end. Now you use AE's point tracker to track the movement of each finger. You apply the track to a null so that the null follows the path of the finger. The reason for the marker is so that you can easily return to the start of the track. Next you copy the position Keyframes. Then you add a new shape layer by deselecting all layers, choosing the pen tool, create the first point anywhere and then paste. The motion path now becomes a shape path. You then add Trim Paths to reveal the line being drawn on.
I guess a third option would be to create the artwork that you want to reveal as a shape layer or an Illustrator file, or as a Photoshop image, import that, then use either of these techniques to reveal it.
After taking a close look at your footage I determined that you probably will not be able to get a good track of the footage and there's also some movement in the shot. Not enough to get a camera track buy you should be able to track the last few frames where your actor is drawing the box tracking rotation and scale, then you would apply that to a shape layer or a graphic of the TV that you wanted to reveal. This is a perfect example of a time when pre-production planning and visual effects supervising on the shoot would have made your job a lot easier.
Here's what It would look like if I were to do the shot. As you can see, the fingers don't draw much of a TV. I'd probably just go with a graphic and try and time the reveal so that it's good enough if you really have to have this effect in the project.
Thank you very much for the information and sorry for the late reply. I am working with your first suggestion. It doesn't have to be tracked by his fingers (Doesn't need to look perfect). I used the solid to trace two paths, one for each finger. And both of them are open ended. I then applied a stroke effect to the "Left Finger" layer as shown in the screenshot. But i cannot seem to make it draw anything. So the path is set and being followed, but nothing is happening. What am i doing wrong?
I don't see a mask path, I see a motion path. You have to draw a mask path with the pen tool on a solid then I would use paint on transparent.