The manuals that come with the Amibroker have some sketchy tutorials, but nothing approaching a structured course so it’s a steep learning curve. Based on my limited experience of tinkering (Read:Copy paste)with AFLs, the best option in these cases seems to be to find some really well commented codes and start putting things together.

Does anyone have any well commented trading systems written in AFL? I do not care whether they are useful systems or not, just so long as I can learn about the BT concepts.

You have tons of very good codes here:
AmiBroker - AFL Library

Also here you have great tutorials on various subjects:
AmiBroker Users’ Knowledge Base

And the user guide is great for looking at specific topics as it is very complete.

Good luck.

You can also check out books by Bandy - Blue Owl Press

Right, Bandy's books are excellent to learn Amibroker especially Quantitative Trading Systems.