Hi and welcome!
Relating tables is pretty straight-forward thanks to cook!
Lets say you have 2 tables - families and family_members each with an id (auto created in cook) and a name field + in the family_members table, add a foreign key field type called family. Then, from your tables list on the left-hand side, drag the families table into the dashed div and click the save button. That's it! Simple, eh?
Now all you have to do is create a layout to enter and view data for both tables. When you do this and you drag your fields to the layout, simply click the + sign on the foreign key field (family) and drag one of the related fields from that table to your layout. This is what you'll see in your layout - either in the combo dropdown on your form or in a grid/fly.
Hope it helps,
Gez