Step 1 is to create the drawing either as an Illustrator or Photoshop file or sketch it as a shape layer.
Step 2 is to reveal the layer using standard masking, track mattes, or in the case of a shape layer, trim paths. You can use a mask path and Effects>Generate>Stroke to reveal the original image or put an animated stroke on a solid and use that as a track matte for your artwork. If you use shape layers as a stroked path with no fill you can simply use trim paths to achieve the animation.
This is very standard stuff. Animating a signature would be a good thing to type in the search help field at the top right corner of AE. You'll find that Search Help is more effective than Google because it usually points you to better tutorials and more reliable information. Trim Paths is another good term to search for from inside AE.
Let us know if you have any other concerns.
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In addition to what Rick has suggested, I also recommend, in your particular case of "free" drawing, to use "paint". Click on your video layer twice to reveal the layer window, and use the paint brush in the tools panel. You can set your writing mode to write-on and it will record your strokes in real-time. (The video will freeze but the strokes will be recorded the way you draw them)
write "paint" in the search field at the upper right corner of your Ae interface to know more.