Where do you actually start if you want to sell on amazon?

Check whether the category is gated before you fall for a product. Some need approval and can ask for invoices from a supplier, so a product that looks great on paper might be one you can't list yet. Also read the 1 to 3 star reviews on the top sellers. That's your list of product improvements, for free.

With the numbers, not the account. Pick a product idea and run it through Amazon's revenue calculator with the real size and weight, because FBA fees and referral fees eat way more than people expect. If the margin still looks okay after a realistic ad spend, then register. You'll need ID, a bank account and a card, and the verification can take a while, so don't order stock until that's through.