It depends on which way you want to associate the data...
Are you storing multiple pieces of data with one Person (first table), or are you wanting to store the same piece of data with many Person entries?
In either case, your main form contains the 'single' piece of data; If you're storing many thing with a person's record, then the main form is the Name fields, Address fields, Phone number, etc...
The subform is based on a table that is related to the main table. It should include its own Primary Key (probably Autonumber), but also a field of the same type as the PK from your main table; Text for Text, Long Integer for Autonumber. Then the fields you want associated with it.
I usually create my subform separately and build it to fit the space in my main form, then insert it using the toolbox Insert Subform/Subdatasheet option. The wizard for this is pretty good about making sure the subform is related to the main form (It will suggest "Show every related record in table2 where PK <=> PK in table1", or something similar.
Then you can select many things in the subform to associate with your main entry.
Does that help?