Erik Kostson
Ansys Employee
For question 1 and 2, we (Ansys employees) can not really comment since we are not designers/structural eng., or civil engineers working with concrete - I am sure though you can find out the properties of your concrete say like (C40/50), or if it is not a standard grade, one can potentially get it measured and get the properties out, but that is not something we do and can advice more on.

For point 3, we can comment since it is related to the use of Ansys products - I would say if you are new to Ansys, use workbench (apdl solvers are used there anyhow when we solve, so it is the same as apdl, but just an easier interface, which is easier and faster to learn than the apdl UI). To help you get started, you could try and follow this course:

The workbench mechanical workflow recommend for RC concrete slabs could be this one (so you can try and follow this - ofcourse this is an example so one would need to change things, e.g., material properties):
All the best and good luck