Please help me how to install Ubuntu/linux OS?

Are you installing it on a machine that has windows on it or or no OS?

Well you can send your PC to me and I will install it and charge you or you could Google "Ubuntu Installion" and find these links.
https://help.ubuntu.com/community/Installation
http://www.psychocats.net/ubuntu/installing

There are literally thousands more links to choose from and some are even videos so you can watch it on one computer while you install it on another.

The short version is put the disk the drive, boot to the disk, and follow the instructions. The explanations are very clear.

For those who've never installed an OS, it can seem to be mysterious, but it isn't. You pick a target drive, create a partition, format it, and tell the installation to proceed. Somewhere along the way the install routine will ask for some information, such as language to use, time zone, logon password, and the like; answer the questions, and, when instructed to, reboot.

Linux distros know they may be installed by persons who have never installed an OS before. Most of them have tried to make to process as easy and transparent as they can.

k3lt01's links will help explain the process in detail and should answer any question you have.