The four divisions

ENVIRONMENT: the bridge to JCL

SELECT PAY-FILE ASSIGN TO INFILE says: the file this program calls PAY-FILE is whatever the JCL supplies on the DD statement named INFILE. That single line is the entire connection between a program and its data, and it is why you can point the same program at test data by changing one DD.

DATA DIVISION: three sections you will meet

Level numbers build a hierarchy

       01  CUSTOMER-RECORD.
           05  CUST-ID           PIC X(08).
           05  CUST-NAME.
               10  CUST-FIRST    PIC X(15).
               10  CUST-LAST     PIC X(20).
           05  CUST-STATUS       PIC X(01).
               88  CUST-ACTIVE              VALUE 'A'.
               88  CUST-CLOSED              VALUE 'C'.
               88  CUST-VALID               VALUE 'A' 'C' 'S'.
           05  CUST-BALANCE      PIC S9(9)V99 COMP-3.

* then in the PROCEDURE DIVISION:
           IF CUST-ACTIVE
               PERFORM 3000-PROCESS-ACTIVE
           END-IF

• 01 starts a record. 05, 10, 15 nest inside it. The numbers themselves are arbitrary; only their relative size matters.
• A level with no PIC is a group item — it is the concatenation of everything under it, and is always treated as character data.
• 88 levels are condition names: a readable name for a value test. Use them; they make logic far easier to read and to change.
• FILLER names space you do not care about. It is not a keyword you can reference.

PROCEDURE DIVISION: paragraphs and PERFORM

Logic is organised into paragraphs, usually numbered so the numbers imply call order. PERFORM 2000-PROCESS runs that paragraph and returns. PERFORM … UNTIL is the loop. PERFORM … VARYING is the counted loop. Modern style uses scope terminators — END-IF, END-READ, END-PERFORM — rather than full stops, because a misplaced full stop is one of the classic COBOL bugs.