if then statement and creating a floor plate

I have basically 2 questions.

1) How can I define a floor plate in gh? I have to use center point, thickness and size information but I couldn't create it. What should I use?

2) How can I make a if then statement in gh? Is there a spesifict tool or should I write a script in monkey? If I write a script, how can I apply it in gh?

I am just a beginner in grasshopper. Maybe these are basic information, I don't know. I searched but I couldn't reach answers. If you help me I am goint to be soo glad.

Thanks. :)