There's a bunch of threads on this, under the Gear and Accessories and Apparel sub-forms, I believe, with tons of great advice about what to wear, when to wear it, not overdressing, etc.

For me, the key autumn pieces are lightweight long-finger gloves, arm warmers, knee warmers, and a lightweight vest. I'll start using some combination of the above once it's below about 65 degrees.

Mental guidelines: See above. Also, I'll cover my knees below ~65. I'll wear knickers when it's 45-65. Below 45, and I'm reaching for midweight tights and winter boots. I want sleeves of some kind below 65. I want my insulated jacket below 40. I have about 3 different pairs of winter gloves that are good at various temps. I have some ski stuff that does double duty as well- base layers, beanies, buffs, etc.

It's a really individual thing, depending on your personal thermostat. If you're just building up your cycling wardrobe, I'd start with good arm warmers, knee warmers, some long-finger lightweight -to-midweight gloves, and either a vest or a convertible jacket. You can add pieces as needed.