If a choking person becomes unconscious, what steps should be taken?

Learn CPR. I recommend you contact your local red cross or american heart association and take a CPR course. If you go to google you can look up CPR anytime and there are resources where you can watch and learn at home.
CPR. You can read up on this topic all you like, but hands-on training is the best and most effective policy. CPR does teach choking as part of their program.