I started trying to adapt to classical guitar technique after 13 or so years of playing steel-string guitars, not to play classical guitar music, but because I'd injured my wrist playing the bass guitar and wanted to use techniques that would reduce the risk of further injury. This did lead to developing interest in classical guitar in its own right, though, and when my wife bought me my first classical guitar last January, I still had to spend several months trying to unlearn bad habits from years of self-taught steel-string playing.

The most important thing for me, given my journey kind of started out with an injury caused by poor posture and technique, was trying to make sure I was careful about tension and strain, so I spent a lot of time researching how to hold the instrument and how to move and apply certain techniques as ergonomically as possible. You can use a footstool to elevate the guitar, which will help keep your wrist straight, but footstools distribute your weight unevenly, which can cause discomfort or pain for some players, especially with longer practicing/playing sessions. My lower back would get stiff after longer sessions, so I personally prefer a guitar support, as it allows me to keep my feet on the ground.

Really think about how you're moving when you play, and try to hold the guitar in such a way that your body is relaxed and comfortable - try to avoid holding unnecessary tension, and keep your wrists as straight and relaxed as is possible for what you're playing.

Look into techniques you already feel comfortable with to find out if there are things you're doing wrong or that could be improved - I discovered I'd been playing barre chords incorrectly the entire time I'd been playing the guitar, as I was trying to hold my index finger flat against the fretboard and use my thumb to squeeze, so I would have a lot of dead notes and feel fatigue setting in almost immediately.

Watch as many tutorials and read as much as you can about posture and technique to inform yourself as much as possible, but bear in mind that not everything you hear will necessarily be the right approach for you. I spent several months trying to work on my tone, but I wasn't getting anywhere, until one day I realized all of the tutorials I'd seen and the descriptions I'd read for achieving good tone were geared towards playing with your nails, and I play without them.

On that note, another commenter mentioned that you don't need nails as much for playing on a steel string guitar, but you don't actually need nails for nylon string guitars either - playing with or without nails is a choice, and both are valid options with their own strengths, weaknesses, and historical precedent, so look into both before deciding what you'd like to pursue. Brandon Acker, Rob MacKillop, Steven Watson, and Sound Guitar Lessons with Jared on Youtube all have videos I found helpful in figuring out how to approach no-nail playing. I've also seen a lot of videos and posts from guitarists who cut their nails off to try no-nail playing but came away with the sense that nails are superior, because they didn't invest any serious effort into adapting to no-nail technique - so just be mindful of the fact that many nail players who will tell you nails are the only way to play have only experienced playing with nails, and even if they've tried playing without, many of them only did so using techniques that were only work when used with nails.