CentOS as a Guest OS in VirtualBox

Download CentOS ISO image
You can download CentOS iso image from its website. No need of burning CD or making bootable USB drive.

Download Oracle VirtualBox and install it
You can download VirtualBox from its website according to your current OS like .exe, .rpm, or .deb etc and install.

Make a new virtual machine
After finishing installation, launch virtual box and create new virtual machine.

1. Press “New“, Type as “CentOS 64 bit” or anything you want. Set the type as “Linux“, Version as “Red Hat 64 bit“. And press “Next”. 
2. Allocate “Memory ” to 512 Mb or 1024 MB . You can increase this value according to requirements. And Press”Next“. 
3. Then select “Create a virtual Hardisk Now” and press “Create“. 
4. Then choose “Dynamically allocated“. Note:- You can choose "Fixed size" also according to your requirement. 
5. Choose the amount of hard disk space you want to allocate for your CentOS system. Select atleast 8Gb-10GB . I have allocated 15GB. And press “Create“. 
6. You would see the Virtual Box Home Screen. Now Press on “Start“. 

Installing CentOS
After creating virtual machine, following step for installing CentOS.

1. After pressing "Start", click on the small folder icon and select the location of iso image file and press "Start". 
2. Once you click start you virtual machine starts up for the first time in a new window. 
3. Choose language and press "Next". 
4. Select the model of the keyboard or just leave it to the default value. then press “Next“. 
5. Select basic storage devices as we fall under that category. then press “Next“. 
6. In the next screen it will ask to discard any data that exists in the hard disk that you created earlier for this VM. Go ahead and say yes to that. Note : This is not going to discard any of your main hard disk data. Only discard the VM HDD drive that you created along the way which should be empty any way. 
7. Name your new CentOS VM Server. Name to what ever you want this server to be called as, then press "Next". 
8. pick a time zone for the Cent OS. then press “Next“. 
9. Enter a password for root user. If you are a newbie to Linux, root user is like administrator.then press “Next“. 
10. In this screen you can choose how to layout your partitioning schema. I am not covering how to change default partitioning schema[Custom partitioning] here. Pick the default option "replace existing systems". This will clean wipe the VM HDD one more time and layout partitioning schema to install CentOS. then press “Next“. 
11. Go ahead and write changes to disk and Cent OS will start installing in your Virtual Machine. 
12. Congratulations ! you now have successfully installed Cent OS Linux Virtual Machine Server. 

Note:- Do not forget to eject the iso image file in settings->storage.