I have installed Windows Server 2012 in VirtualBox inside iMac. Now I need to access the website created in VirtualBox in iMac. can any one guide me please?
It is too easy. Firstly you just need to set the virtual machine to use bridged networking, so it will get an IP address on your network. This IP address will be accessible from your host machine.
According to the Chapter 6 of VirtualBox's manual:
With bridged networking, VirtualBox uses a device driver on your host system that filters data from your physical network adapter. This driver is therefore called a "net filter" driver. This allows VirtualBox to intercept data from the physical network and inject data into it, effectively creating a new network interface in software. When a guest is using such a new software interface, it looks to the host system as though the guest were physically connected to the interface using a network cable: the host can send data to the guest through that interface and receive data from it. This means that you can set up routing or bridging between the guest and the rest of your network.
You can also use the NAT networking mode, but you have to work a lot to set the port tinkering.