The first question you should ask yourself is what you want to achieve, beyond just saying "to be fit", and be honest with yourself. Do you want to be stronger, bigger, more flexible, better endurance, etc...?

If your main goal is to just stay overall fit and flexible as you age, yoga is a great inexpensive option that you can do at home without fear of other people judging you (if you got social anxiety). Get a yoga mat and try out a yoga session, you might get pleasantly surprised. Down Dog is a great app that generates sessions for you based on your time, experience level, etc...

By the sound of it though, you are currently looking at training in a gym doing weightlifting, since you're mentioning stance and bracing. In that case the best advice would be to stay the hell away from fitness influencers while you're getting accustomed to the gym.

The industry is rife with grifters that are making a living criticising all sort of things and telling you to DO THIS THING IF YOU FEEL THIS TOTALLY NORMAL THING DURING YOUR SET (looking at you Squat University) because they know many people who go to the gym are insecure since it's a completely new environment and prefer to be told what to do.

Simple fact is that there is no one size fits all. People are built different which means your friend with a short torso and and long legs will need a different stance than your friend a long torso and short legs, etc... Unless you got a competent PT to help you, you'll need to find out yourself what works for you.

Find a beginner program that fits your goals and schedule and stick to it, and don't worry too much about doing the movement perfect. It's takes time for your nervous system to get used to a new movement. Don't expect your deadlifts and squats to move perfectly, it takes time and repetition. Experiment with stance width, grip width, etc... to find out what works best for your body.

And remember that your body is more resilient than you might believe. Don't worry too much about your aches and pain, being sore is normal and not harmful, often your brain will play tricks with you and makes it worse the more you worry. Barbel medicine (which is run by actual medical doctors) is a good source if you got fitness influencer induced exercise anxiety.

Good luck on your journey and remember, the most difficult mile is the one from your house to the gym :)

Edit: some links to help with the technique/bracing confusion you got.

A level headed video talking about lifting technique.

How to brace, explained by the current world's strongest man (bad/clickbaity title unfortunately)