Hi psarakia. This isn't the right place to start that kind of discussion but I can point you in the right direction.
This website has a program that can give you a good estimate of how much substrate you need. It is the Substrate Calculator and it's located here:
http://www.plantedtank.net/substratecalculator.html
Either substrate grows plants well so pick whichever looks best to you. If you still have questions, you can start a thread in whatever forum is appropriate and you should get good answers.
The Search function is also very useful.