Watch the tutorial here: https://www.youtube.com/watch?v=9Qz4Nldjv6I
First, draw your line. You can delete the fill, change the color, and more using the shape layer's dropdown menu in the panel.
Now, select the path. Open the "add >" menu in the panel and select "trim paths."
Edit the start and end times to your liking using keyframes. (At the beginning, your end point should be 0%.)
Change these two keyframes to easy ease, and voila. You have now animated a line!