It is doable, but definitely requires to get used to the basic functions and concept of the program. How to do it depends on your shot.
Do you have a static camera? This makes everything a lot easier, otherwise it becomes quite complicated.
To get started:
1) Draw the line using a shape or solid layer.
2) Animate the layers position using keyframes to follow your player.
3) If you want the line to be placed "below" the players, try to use color keying techniques to get a version of your clip where the green grass is transparent and put it on top of the line.
4) if you want the line to follow the player automatically, you can experiment with tracking, but it will be hard to come up with an automated solution that is faster than doing it manually.
Yeah, OK I won't lie. No idea how to do most of that!
Is there a tutorial I can go through to get used to the basic functions first? What are keyframes? I had a quick go with tracking manually (placing the tracking points to follow the player) but didn't know how to get the line to follow it (or how to draw the line at all!).