If you’re primarily going to be promoting Amazon products it’s important to choose a set of products that has a relatively high price point. After all, it’s hard to make a solid income off of promoting $1-5 products, since the commission on Amazon is so low. Items that have a higher price point, like high end appliances, furniture, BBQs, blenders and juicers, or bikes can net you over $50 a sale at least.
You can sign up as an Amazon associate straight away without a site. As long as you have the URL and it belongs to you. They won’t approve your site until you have made your first commission. So what I would do is get the site built and add all the content that you need. Make sure its finished. Then sign up to the Amazon associates, add in your aff codes to your review pages and then you just wait for your first sale. Make sure you read the amazon T&Cs so your site is compliant. If it isn’t then they will not approve your site.