New seller

It would take us days/weeks to give you essential advice, but first of all make sure you thoroughly read Amazon's 'Seller University'.

Also, assuming you are in UK, then you MUST be registered with HMRC as self-employed (possibly sole-trader to start off with), wait to receive your UTR (if you haven't already), and have a BUSINESS seller account on Amazon (which is not the same as professional selling plan).

It will depend what category of goods you are looking to sell, some are gated, some require permission, and some have very strict labeling and safety requirements, esp. baby items and toys.