Kdenlive is a free to download open source multi-platform video editing software.
Assuming you require the Debian distribution package, follow these simple steps.
To get started, open your terminal with:
Ctrl+Alt+T
Then, run the following commands one line after the other:
sudo apt-add-repository ppa:kdenlive/kdenlive-stable && sudo apt-get update
sudo apt-get install kdenlive
That's all you should need. Once the installation is complete, you should be able to open and run the program.
Alternatively:
You can install it through the software centre, just search for "Kdenlive" in the software centre, and click Install.
Related: How to Install Lightworks on Ubuntu