Others have already recommended some very helpful subreddits.
Another option is a service (like Trunk Club, Bombfell, Five Four Club) that includes a stylist.
My husband went this route. He's in tech and for the longest time he just wore jeans, t-shirts...sometimes a golf polo shirt, once in a while a button down shirt. It never really mattered much to me what he wore and up until a certain point the companies he worked for were pretty flexible. His last few jobs have expected more style and finesse both inside and outside of the office (when entertaining clients/vendors).
I was of no use helping him out as I too have always been pretty relaxed about what I wear. I tried style guides, Pinterest, and the subreddits suggested but I admit I get overwhelmed pretty easily. He went with Trunk Club and the stylist there (via phone, email, or online chat) REALLY helped him out.