Step 1: Download the latest Ubuntu ISO http://www.ubuntu.com/download/desktop/ You can choose either Ubuntu 14.04.1 LTS or Ubuntu 14.10, choose the LTS version if you want a really secure, stable version.
Step 2: Acquire a USB stick or a Black CD/DVD. You can then follow this guide to make a Ubuntu installer on the USB or DVD. I recommend you use a USB, much faster than a CD/DVD and they usually result in fewer issues.
Make a CD/DVD: http://www.ubuntu.com/download/desktop/burn-a-dvd-on-windows
Make a USB: http://www.ubuntu.com/download/desktop/create-a-usb-stick-on-windows
After you have done this backup any important files that you have.
Reboot your PC and while it's booting mash the 'F' key that brings up your boot menu, for most computers it's F12, F10, F4 or Backspace but it could be any 'F' key. Select either the USB stick or CD/DVD and the Ubuntu installer will start, from here you can follow this guide http://www.ubuntu.com/download/desktop/install-ubuntu-desktop
I recommend you 'Download updates while installing' and 'Install this third-party software' to do this simply have your PC plugged into a wired connection, if you are using WiFi in the top right corner you can select your WiFi there and connect to a network, I recommend you connect to a network first thing. When you get to the storage section just select 'Replace Windows 7 with Ubuntu' finish the installation and there you go, enjoy Ubuntu. I hope this helped.
P.S. Don't be put off Ubuntu 14.10, It is stable but if you want the most rock-solid version choose 14.04.1 LTS, if you want the newest features and technology choose 14.10.