Working specific muscles is a terrible idea for the typical person looking to improve general fitness. The major compound movements are almost always what you want: barbell squats, overhead press, bench press, deadlifts, barbell rows, chin/pull ups, and dips[1]. Those will fix your posture problems among other things. From there you can work your way up to showing off on rings, glute ham raises, standing back bends, and even a planche progression if you really want to break the ice at parties.

I mean sure, if you’ve got the genetic gifts, the drive, eight hours a day to spend in the gym, and probably the gear to be the next Mr Olympia and you just need some hypertrophy on those posterior delts to put you over the edge then by all means target specific tiny muscle groups.

On the other hand if you just want the usable strength to handle life’s unexpected challenges then compounds all the way.

Edit: If you're a true beginner, get a good coach. Frankly I think beginners would be better served by a site on how to identify a good coach than one on how to do the movements.

[1] If you're a beginner you likely won't be able to do pull ups or dips unassisted, so either find a gym that has a machine that can give you assist, or use rubber bands. In fact if you're truly out of shape you might not even be able to do a proper bodyweight squat. If so, no problem, you can either assist by using rings, or progress with a leg press machine.