Here's a few basic pointers to help you get started.

Work out what you want to sell on Amazon and seek out the bone fide supplier, you'll need to work with either the maufacturer or their official wholesaler and not buy from shops or online retailers. The easy test is if you can buy the item from them without prior verbal/email contact (i.e. by buying from their shop or website as anybody else can) then they're probably a retailer rather than a wholesaler and their invoices won't be accepted by Amazon.

From there be sure you have the agreement of the brand owner for you to sell the product on Amazon in writing before you commit to buying. Wholesalers will have a minimum order which could be tens, hundreds or even thousands of the product, it usually depends on the value and/or physical size of the product and you may get a discount by ordering more. Make sure the invoice is made out to the legal entity you have listed with Amazon seller central. If possible check you'll be allowed to sell the prodcut on Amazon by applying before you purchase but often that isn't possible as you'll probably need an invoice just to apply.

Decide whether you have the storage space, capacity, packaging, labeling, courier contract etc to fulfil the orders yourself from your own warehouse/garage/home or whether you (and your customers) would be better served by sending them into Amazon for FBA fulfilment. FBA has a lot of advantages but if you want to sell mulitchannel eg via your own website, eBay, Etsy, Facebook, TikTok shop etc then its much better to have full physical control of your own stock. Customers are used to and expect quick delivery on Amazon so if you're based outside the UK then FBA maybe the best/only option. Dropshipping is possible on Amazon but you'll need written agreements in place with suppliers before you can list their products on Amazon.

Read up on Amazon guidance for anything you're unsure of by using the search facility in seller central and only believe videos posted by Amazon Seller University, the rest are posted by people who make their money from YouTube and TikTok views rather than as real Amazon sellers.

Understand your tax implications both personal and coorporate, e.g. if you're based outside the UK you'll have to charge your customers and pay VAT straight away, if you are a UK resident you'll need to register at £90k annual turnover although occassionally it is better to register below that depending on your business model. An accountant is best placed to advise and help you keep the tax and financial side of the business straight.

Once you know your input costs, Amazon fees (all of them), taxes, professional fees and fulfilment costs, have thought about things like product liability insurance and built in a realistic margin for breakages/items lost in transit/unsaleable returns etc and have worked out how many of the product sells each week/month on Amazon then run your numbers to see if the business will give you a profit if you sell at or match the best price on Amazon. Make yourself aware of any potential hazards/selling restrictions there are with your product eg bladed items/internal batteries/most childrens items/many cleaning products/food items/alcohol to name just a few have extra restrictions you'll need to overcome and get approved to sell. Time is a big consideration too, if your product attracts lots of returns or customer questions that all has to be sorted too.

There's a lot to consider when you first start but once you've been through it with one product then adding your second, third product etc becomes easier. Few people make a living from just one product so you'll probably be selling a range of products before you're making enough to call it a decent living.

Selling on Amzon is tough, it's full of rules/regulations and metrics you need to keep on top of. Fall foul of them and your Amazon journey maybe restricted or cut short. Incorrectly source the products you sell on Amazon then in some circumstances Amazon will keep your funds and destroy your FBA inventory for you at your own cost. Do it correctly and it is possible to make money otherwise no one would be doing it.

Oh and while some people will tell you otherwise we've lost count of the number of people on the forums who have bought items from Amazon themselves in the UK, US or any other country (inc Amazon Business) and then tried to the sell the same items on Amazon who have had their invoices rejected, funds withheld, inventory destroyed etc. Amazon consider themselves to be a retailer which goes back to the 'how to buy your stock' test at the very start.

That hopefully answers your question. Most things beyond these basic pointers will be quite product specific, eg does it need to be UKCA marked or will you be required to submit safety data sheets etc etc but your supplier maybe able to help there if they already supply into or are familiar with the UK market. Just remember if you are the seller then you are responsible so check out anything you get told.

Good luck and happy selling.