In Ubuntu there is an option : Install Ubuntu alongside Windows Boot Manager. So don't worry about losing data on Windows but backup your data before installing Ubuntu:
2. Then Download Ubuntu.iso file (version 17.10, or any)
https://ubuntu.en.uptodown.com/ubuntu/download
3. Install Oracle Virtual machine from step one.
4. Prepare the VM for windows installation by adding new operating system based on the request fill the information. Note that your aim to install that Linux (Ubuntu) give he name.
5. show the path where is the iso file Setting--> Storage
6. After that go back to start on VM then click or show arrow at the top of VM