I started doing Assembler for IBM 7040 IBSYS. I did start relatively "simple" in '65, but advanced fairly quickly.
I started OS/360 Assembler in '68. As with 7040 I tried "simple" stuff first, and advanced to more complex stuff rather less quickly than with 7040. As with the 7040 numeric conversions were somewhat difficult. One project in early '69 I still recall with pride was to build a macro set that linked to the Fortran library for reading and writing. The macro set would build "compiled" Fortran formats.
Your proposed Hello World program requires "simple" macros like SAVE and RETURN, as well as some of the "simpler" I/O facilities like OPEN, CLOSE, PUT, and the base parts of the dreaded DCB macro.
WORLD CSECT DEFINE PROGRAM CSECT
USING *,12 SPECIFY PROGRAM ADDRESSABILITY
SAVE (14,12),,* SAVE REGISTERS
LR 12,15 COPY ENTRY POINT ADDRESS TO REG 12
LA 15,SAVEAREA LOAD ADDRESS OF THE NEW SAVE AREA
ST 15,8(,13) ADD THE NEW SAVE AREA TO THE
ST 13,4(,15) SAVE AREA CHAIN
LR 13,15 ESTABLISH THE NEW SAVE AREA POINTER
OPEN (PRINT,OUTPUT) OPEN SYSPRINT
PUT PRINT,HITHERE WRITE HELLO WORLD
CLOSE PRINT CLOSE SYSPRINT
L 13,4(,13) LOAD ADDR OF THE PREVIOUS SAVE AREA
RETURN (14,12),T,RC=0 RESTORE REGISTERS AND RETURN
SAVEAREA DC 9D'0' NEW OS/360 SAVE AREA
PRINT DCB DSORG=PS,MACRF=PM,DDNAME=SYSPRINT,RECFM=FBA,LRECL=121
HITHERE DC CL121' HELLO WORLD!'