There are a few things you must do.
There are some things you should do.
There are many things you can do.
You must stay physically active as you get older. This comes in many shapes and sizes. Pick something you love and do it.
You should strength train. The research overwhelmingly supports resistance training as people age to improve quality of life and many health outcomes.
You can do lots of things to get stronger. I personally prefer barbell training, but machines, dumbbells, cables, bands, bodyweight, etc.. all can provide a stimulus for growth. Pick one and improve over time.