Branching works slightly differently to how you would expect if you are coming from other Git based tools.
In Trunk we automate the branching. Any changes that you make to a file are automatically stored on your own private branch. These changes can only be seen by you.
Once you are happy with the changes you have made, you can commit the changes to the master Trunk. Doing this makes will make your changes visible to the other members on your project.
It's best to treat the Master branch as the source of truth for your project.
See also inviting members to Trunk.