Ubuntu on different partition than Windows
It's not possible to install two Operating Systems on one partition. Either you replace it on the partition where you have a different OS by choosing that partition for your install, or you Create a new partition and install the different OS on the new partition.
You mentioned that you know how to create a partition. So perform that step and install the different OS on the partition you create for it.
The Installation
Installing Ubuntu is a very seamless operation. Just boot to the install DVD or the USB installer. You'll be prompted to either Install Ubuntu or Try Ubuntu. Follow the prompts and it'll be installed on your system.
If you already have an Operating System, as in your case, when you run the installer the install options will include install
- Alongside the current installed OS. With this option the installer will bring up a slider and allow the user to determine how much space to leave for the current OS and allocate for Ubuntu. With this option you don't have to create a partition. The Installer will split the partition for you, having one for your current OS (Windows) and the other for Ubuntu.
- Use the whole Disk for Ubuntu. This option would clearly remove everything currently on the drive and replace it with Ubuntu.
- Do Something else. On this choice you will be given an option to select a partition where you want Ubuntu installed. This would be the option you would choose for placing the OS on the new partition you mentioned in your Question that you know how to create.
Note
The installer will clearly notify with a confirmation prompt before making any changes to your system. It'll tell you which changes will be made.
The process is probably easier done than said.
Official Install Steps with pictures:
https://www.ubuntu.com/download/desktop/install-ubuntu-desktop