"Content" not appearing in layer properties?

Now, more than ever, you need to be very careful about the tutorials you follow. Most of the new ones on YouTube are posted by enthusiasts that have found some recipe that works for their specific task. Most do not explain things well or promote and encourage efficient workflows. That's precisely why the tutorials you found online did not give you enough information to use a basic shape animator tool. The User Guide and the Learn Workspace are the first places to start learning AE. The Search Help field in the top right corner of AE is almost always a better source for answers and tutorials than Google or YouTube. Searching Trim Paths will give you this list of resources and this decent tutorial on how to use the tool is right there at the bottom.

If you ever have a problem with a tutorial, make sure you include a link so we can help you figure out what info is missing.

And if you need to create shape layers and add Trim Paths to them, you can do it this way:

1) Activate the pen tool.
2) Without having any layer selected, make your line and give a Stroke width. A shape layer will be created.
3) Remove the fill.
4) Go to the layer content and Add a Trim Paths.
5) Modify it according to your needs.

Trim Paths is a shape layer animator that can be added to a path with a stroke. You have to convert your AI Vector Layer to a shape layer using the Layer>Create menu. If you used a custom brush on a path, or you have a closed path with a fill trim paths may not work as you expect it to. 

If you have any other problems please embed your screenshots so we don't have to download them and make sure to show the modified properties of the layer that is giving you problems by pressing the U key twice before you take the screenshot.