Samid,
Depends upon what you want. If you just want APO, nothing additional is required.
Like many SAP solutions, you can optionally activate additional specific Business Functions, but these are not a requirement just to get APO up and running.
You will need to create a LiveCache instance; which is almost always on a separate server. If you wish to implement either Optimizer or Capable-To-Match, you will also need an optimizer server. I suggest that you don't implement the optimizer unless you know up front that you will be needing it (you will discover this fact when you gather your business requirements).
All of this information is covered in the installation guide. Rather than waste further time in this thread, why not just download the IG and begin to study it. You will need to become familiar with it anyway before you begin your APO journey.
Best Regards,
DB49