If you do not already have a git repository for your dbt project, you can let dbt Cloud manage a repository for you.
To create a managed repository, choose the "Managed" option when creating a repository in dbt Cloud. Next, supply the name for the repository -- something like "analytics" or "dbt-models" is a good choice.
Once saved, dbt Cloud will host and manage this repository for you. If in the future you choose to host this repository yourself, you can contact support to have the contents of your repo transferred to you.