Correct. In a 172, there is only one pilot, so one PIC, so one person logs the time as the pilot. An instructor providing instruction may allow the student to log their flying time on that flight as dual instruction, which means that the student is not logging as PIC. Once a student is licensed, they may (and should) log as PIC while they are the pilot of the plane. In the case of instruction with an instructor, discuss with the instructor in advance who is PIC and shall log the time, they will explain it to you - but there will still be only one pilot.
That said, it's your log book, and you can record what you want. But, if you log flying as a non pilot/PIC, you must not include this time in totals in application for insurance/currency/further license. There could be times where as a licensed pilot, you may fly under instruction/supervision, but cannot log the time, until you get that rating. Multi engine/Seaplane being two examples of what you might fly, but cannot log as PIC until you hold the rating.
By the time you get to a type in which you can log "co pilot" time, it will most likely be a type which requires a type rating, and of a commercial nature - you'll know when you're flying it!