Don't apologize! Not a single person here came out of the egg knowing how this all works. We all have to learn. Please feel free to ask any question any time.

There are many methods of calculating body fat. I believe the most accurate is by being submerged underwater, presumably at a gym. You can also have body fat calculated with calipers that you either buy, or have done for you at a gym.
But probably the most common method used is calculating by taking various body measurements. These may not be super accurate, but if you always use the same method you'll at least have an idea of the change (decrease ) in your body fat.

If you search the net you'll find a few, but here's one to start with.

Cheers, matey.

Roz