FileMaker Training Series12 is pretty good. I think it's free from the FileMaker website.
I'm not sure what you are trying to accomplish. If you create a record in A and always want a related record created in B, you might not need B at all. But maybe you do so I might script it Go To Layout, Create New Record, Set Field, etc and then write the two keys into the join table. FTS 12 shows all of this and is a good introduction to simple portals as well scripting these sorts of things. As a bonus you'll learn local and global variables which are handy little devils to have in your toolbox. FMPA 12 is quite diverse on the techniques available for this.
I find myself scripting portals and relationships all the time. Scripts also allow for the use of script triggers so that a lot of stuff can automated and hidden making for the simplest interfaces.
When I first learned FMP I had trouble getting my head around portals. Once I got some good information on learning them I discovered that they are really pretty simple. They look complicated at first because they are extremely flexible in their usages.
Normally, linking a record in Table A to Table B requires creating a new record in the Join table. You would select George in a new record of the join table in order to link the record in Table B to the current record in Table A on your Table A layout. Since Table A could be linked to many records in Table B, this is done with a portal to the join table where (in many cases) you select from a drop down list or pop up menu in order to select a record from Table B to complete the new portal record and establish the needed link.
This is what takes place in the Invoices Join table when you select a product in the portal in the invoices layout. You are creating a new record in Invoice Data--the join table--that links the current Invoice to a specific record in Products.