Hi @vegforlife thank you for your great question! I'm a firm believer that different methods work for different people, in all aspects of life, including creating success for yourself with Amazon.
I think its important to mention first that there are several ways that someone can sell on Amazon. An example is Fullfilled by Merchant (storing inventory and shipping/ handling customer service yourself), or Fulfilled by Amazon (ship all of your inventory to Amazon and let them take care of the rest). You can also elect to sell an existing product or create a unique product to sell. Theres also the option of opening a professional seller account or a regular one.
There are pros/cons to each of these methods and entire books have been written about them. I'd be happy to go in to greater detail with you and help you discover what works best for you if you'd like. Just shoot me a message and we can talk through it. I can talk about Amazon for days and dont want to completely hijack this comment thread :) Either way, in my humble opinion, an important first step is discovering which methods are best suited for YOU and the Amazon business you want to create.
As far as the biggest one piece of advice...I'd have to say that its this: Be prepared to make the customer priority #1. In the age of online sales, customer reviews will ultimately determine your success rate. Taking great care of every customer every time is paramount. Of course, there are several other factors to consider and I'd be happy to discuss them with you if you'd like. Hope this helps and I wish you great success in your venture!