LPT Request. My body should not feel this weak in my 20s.

Much like other American millennial, my late childhood and early adulthood physical education consisted of either running a mile or learning weightlifting, which does not translate to a wholistic physically active life as an adult.

I am 29 years old and I can feel my body slowly getting creaky. I would love to fix my lack of knowledge before my body decides to stop learning new tricks.

If you have any links or advice on proper physical exercise I would love to learn. I am talking, how to run properly so that my knees don't ache, how to stretch properly before and after physical activity, best ways to improve flexibility and endurance, etc.

Thank you :)

Edit: obligatory wow I did not expect this to blow up like it did. I'll do my best to reply to everyone. I want to say thank you so much! There is so much advice and encouragement in all of your responses, I am honestly overwhelmed.