You've got two different issues and the choices you make in dealing with one will afect the other...
On the one hand, you need to design a database with fields and relationships that support your project. FileMaker is not a desktop publishing system nor a word processor, but can be useful in cross referencing and in organizing the structure of your document's content...
One the other hand, you want to set up easy to use tools for searching out the data you need from those fields and records in your database file.
I can share a few pointers, but can't really advise you in detail as that would require knowing a great deal more about your database project.
IF your text is entered in a single field, but you have potentially many records (each record can be one paragraph, one chapter, one topic, etc...)
You can enter find mode, type "intro" into the field, perform the find and pull up all records where the records found are all records where intro appears in the field at least once.
If you want to find all records that have intro AND apple, you can enter:
as your search criteria.
If you want to find all records that contain intro OR apple, you would:
enter find mode, enter intro into the field.
Select New Request from the requests menu.
Enter applie into the field
Perform the find.
All of these searches can be done with scripts as well as by the manual method I have described.