You would most likely be looking for how to install ubuntu. There are countless guide scattered about the internet but here are some links to some easy guide to it
http://www.ubuntu.com/download/desktop/install-ubuntu-desktop
https://help.ubuntu.com/community/Installation
Some things you will need beforehand, The operating system on a DVD or on a USB that is the big things you need to use these guides above. I hope I helped you with this, if you have anymore questions just ask all over the site.
(Also you dont just "Access" Ubuntu from your computer its a process that involves rebooting your computer, Windows 7 is an operating system, While ubuntu is also an operating system, Ubuntu however is free and does not cost large amounts of money like other operating systems. Also Do you just want to get ubuntu for anything in specific? As ubuntu is a very hard operating system to use if you have no other knowledge of it. Ask around on the site if you have any more questions about ubuntu) P.S Appropriate tags for this would be ubuntu installation or questions