I'll restate your question with the terms you would look into on the FMP help index.
You want a parent table and a Child tablerelated by a key field in each.
You want to create a new record in the parent table.
You want to create a record or multiple records in the Child table through the relationship.
You then would like to perform a Find, or make a subsummary report listing the parent record and the associated child records.
On your layout, you could have a portal that shows all related records from the child table in a list.
Build the two tables under File>Manage>Database, put a keyfield in each table (autoenterID on the parent side), link them through the relationship tab. Double click the relationship box on this same tab and check "Allow creation of records via this relationship" on the child side only.
On your layout based on the parent table, put in a portal based on the child table.
Does this cover your whole question? If not, please let us know which aspect of the above, or which further step beyond it you are getting hung up on.
Enjoy the day!
Much appriciate it.