Step by step:
Now Open Virtual Box.
Go to Centos mirrors:
NOTE: For Centos 7 go to: http://isoredirect.centos.org/centos/6/isos/x86_64/
Choose the mirror you want.. and you will see a list of isos and torrents to download:
Then Download the two *-binDVDX.iso files ... or by torrent if you feel more comfortable.
Create a new vm, the wizzard will let you choose the memory and storage to assign. then finish the wizard
Once the files are downloade goto to the setting s of your VM, and on storage-> Storeage->tree look for the iso of disk1 (by clicking the small disk icon with a arrow to display the option to select from disk the iso).
Then start the VM, the installation process will start, just follow the instructions and enjoy.
P.S. Do you pay attention to the flags?