Hi Lavinia! Welcome to Gen3 forum
Availability mechanism is a mechanism by which the project will be made available to users, for example, dbGaP access. It is not required field and you can skip it. The description of every field should be available in your data dictionary page. The link to the data dictionary viewer looks like this: https://your-commons-name/DD. Page has two views: Graph View and Table View. In the Graph view you might click on the node and then click "Open Properties" and see all required and optional properties with their descriptions and download JSON or TSV template for submission. On the Table view you find node by it's name and purpose (Administrative, Clinical, etc) and by clicking on the node see same information with properties, descriptions and possibility to download JSON or TSV template. Please see this page for more information https://gen3.org/resources/user/dictionary/
Typically before creating Core Metadata Collection, operator first creates program and project. Please see this documentation about programs and projects https://gen3.org/resources/operator/#6-programs-and-projects. If you create project inside a program (adding program URL to the /_root/), you don't need to fill program's required information in the project's properties, it will be added automatically.
Here you can find examples for different nodes https://github.com/uc-cdis/datadictionary/tree/develop/gdcdictionary/examples/valid, but it is always better to refer to your dictionary viewer and TSV or JSON templates, as nodes/properties in different dictionaries may vary and what fits one, can raise an error in another.