I have installed the Virtual Box, and I would like to make a Debian virtual machine but I can't do this because my Virtual Box don't allow me to choose Debian x64. My computer and the .iso file  are x64 too, that's why I don't understand why I can't choose it. How can I solve this problem?
Why I can't set on the Debian 64 bit on the Virtual Box
debian;virtualbox;virtual machine;64bit
You need to create a 64bit virtual machine. The bitness of the host OS is irrelevant, it's the VM that needs to be 64bit. From the VBox website (emphasis mine):64-bit guestsVirtualBox supports 64-bit guest operating systems, even on 32-bit  host operating systems, provided that the following conditions are  met:You need a 64-bit processor with hardware virtualization support (see the section called Hardware vs. software virtualization).You must enable hardware virtualization for the particular VM for which you want 64-bit support; software virtualization is not supported for 64-bit VMs.If you want to use 64-bit guest support on a 32-bit host operating system, you must also select a 64-bit operating system for the particular VM. Since supporting 64 bits on 32-bit hosts incurs additional overhead, VirtualBox only enables this support upon explicit request.On 64-bit hosts (which typically come with hardware virtualization support), 64-bit guest operating systems are always supported  regardless of settings, so you can simply install a 64-bit operating  system in the guest.WarningOn any host, you should enable the I/O APIC for virtual machines that  you intend to use in 64-bit mode. This is especially true for 64-bit  Windows VMs. See the section called Advanced tab. In addition, for  64-bit Windows guests, you should make sure that the VM uses the Intel  networking device, since there is no 64-bit driver support for the AMD  PCNet card; see the section called Virtual networking hardware.If you use the Create VM wizard of the VirtualBox graphical user  interface (see the section called Creating your first virtual  machine), VirtualBox will automatically use the correct settings for  each selected 64-bit operating system type.So, just try creating a new Virtual Machine and make sure it's 64bit.