I need to install Hyper-V on a Virtual Machine to have different OS to try my programs.My Virtual Machine is Windows Server 2008 R2 but if i need to i could change it to 2012.Could you explane it relatively simply because i'm not an expert.
What you're looking to do is to run a "nested VM". I suggest you use this term often when Googling for a solution. Nested VMs are usually not particularly straight-forward, and a highly-dependent on several things specific to your configuration - physical hardware, virtual hardware, OS, virtualization softwares, etc. You might imagine that running a 64-bit VM on a 32-bit hardware platform can cause problems that are "challenging" to overcome.
Here's VMWare's thoughts on nesting VMs.
Depending on what kind of VM you're running, and what kind of Hyper-v you want to run, you should look for configuration-specific instructions.