What are your opinions for business casual? New job

I know there are a lot of differing opinions on "business casual" and what it means. My new workplace is a mix of polos and button-downs and chinos or slacks. I think it's probably technically more "casual" than "business casual", but the official dress code isn't too specific.

I would like to hear your opinions, rules, and advice.

I generally would prefer to wear a button down. Tie/no tie? Do you have any rules about patterned shirts and solid ties? Which color pants with which color shirts? Darks or lights? Any must-have for my wardrobe?

I feel like I have so much freedom to choose it's slightly overwhelming. I'm coming from military where I wore the same thing every single day for years so I've never really had to choose before.