I believe that laptop comes with ubuntu 16.04 to download that OS see this download page you will probably want the "ubuntu-16.04.4-desktop-i386.iso" but you will have to decide that based on your needs and computer.
To install the OS see this tutorial there are many other tutorials online if you prefer a different one.
Edit:
It looks like there is some confusion over whether Dell uses its own version of Ubuntu or not (see comments). Either way, they have a Ubuntu iso 12.04 on their website see 12.04-OSP1
Edit2:
OP just wants to install Ubuntu.
To set up just Dual-Boot just follow this guide: How to Install Ubuntu Alongside With Windows in Dual-Boot | tecmint.com
To just Install Ubuntu see.
How do I install Ubuntu?