This method will remove all your data and previously installed operating system. So, make sure you have backed up your data.
You can download CentoOS Linux from https://wiki.centos.org/Download and create CD or USB of CentOS.
Reboot your PC and Open BIOS. In BIOS, set first boot device as CD or USB stick or any other drive in which you have CentOS Linux. Save your BIOS setting and boot your system.
Now, your CentOS Linux installation will start. You can see instruction on your monitor about Loading of CentOS Linux. Follow step by step instruction given by CentOS.
Dual Boot Method
If you have other operating system like Windows, you can install Linux as dual boot method. In dual boot method you will install Linux on the free disc space of your computer primary hard disc. You will get option to choose operating system when you will boot your computer.
In this method also, you need Linux CentOS CD or USB hard drive or Stick.
In Windows, go to Control Panel, then Administrative Tool and Choose Disc management from Storage Section. Select drive in which you want to install Linux. Right Click on the drive and choose Format Option. Make sure that you are not selecting the drive in which you have installed any OS or other software. Backup data from this drive before formatting.
After the partition is formatted, right click again on the same drive and choose option Delete Volume. After deleting, it will be shown as free space.
Restart Your PC and Open BIOS. In BIOS, make your first boot device to your Linux installation CD or USB stick. Save your BIOS and boot your system.
Installer of Linux will start and it will ask where to install Linux. Choose to install on the free space of your hard disc. Follow the instructions to complete the installation. Then re-boot your system.