Git is the tool that helps you keep track of your changes. If you have no used Git before, I highly recommend taking a look at this free tutorial:
If you are just starting Gitlab CI, as soon as you have created a new project, you will be asked to add an SSH key.
You can still do changes to the repository WITHOUT doing this step.

You are free to skip it for the moment but if you want to push changes from your own computer, you will need to complete this step.
> Video tutorial
> Step by step guide
> Video tutorial
> Step by step guide
> Video tutorial
> Step by step guide
Please let me know if you need assistance. I am here to help.