How can I find out if my installed OS is a 32-bit or a 64-bit?

How can I find out if my installed OS is a 32-bit or a 64-bit?

I am a newbie so maybe my question is not so shiny. I wont to install
something (virtualbox) but I have to chose between the 32-bit version and
the 64-bit version according to my OS. Problem is that I don't know/don't
remember what type of OS it is from that point of view.
So my question is the following: Is there any fast and easy way (for a
beginner like me) to find out what kind of OS is using? Thank you!