PLC coding/ladder daiagrams (GX Dev)

Anyone have knowledge on these and creating ladder diagrams for a simple PLC set up? Need some help with some fixes for a small (my first) program.

I've been using MELSOFT's GX Developer (Mitsubishi).

 

Post your code and question.

 

Kinda hard to type it out so I'll post the best images I have.

http://puu.sh/gQRKW/69bd1e5da3.png

It's essentially a car park with 6 parking spaces. There is a Start button, an emergency stop button and an off button (which only works when the car park is empty). There are two barriers, entry and exit. Both barriers have two sensors, the first sensor opens the barrier, and the second sensor closes the barrier. There is a neon sign for "Spaces" and for "Full".

The issue I have is that I can put 6 cars in and take 7 cars out, which shouldn't happen. Also was wondering how I could make the off button disable the entry barrier, but keep the exit barrier active until all the cars have left and then disable it too.