The starter won't turn over, but the interior lights are dim. Its getting cold outside.
I've read about the immobilizer, has it already happened. What are my options?

The battery is likely bad, if it was dry. Remove the battery and take it to an auto parts store for free testing. If it's salvageable, they will charge it for free. Personally, I would install a new battery. Most new cars only come with a 36-month battery.

Get a new battery. When you buy a new one get a sealed unit so it does not go dry.