If you have decided on your niche, then you have to find the best products to promote for it. Choose a couple of main products ($100+) and then several smaller related products, that people tend to buy as add-ons, or extra peripherals to complement their main buy. This increases your sales count faster, and thus your commissions bracket.
If you're creating a wordpress site, then there are tonnes of plugins you can use to set up an Amazon Affiliate site.
You need to also find the best keywords to target for that niche, and whether the competition is beatable. And of course, you need good content. Spend some time polishing this aspect. Make your articles enjoyable to read.
If you have some money to spend, then consider buying the Azon Affiliate Toolkit. Does almost everything ->product research, keyword reasearch, stats, competition, article scraping, product images, reports etc.
If you have no money to spend then get out that excel sheet and spend time on finding the best keywords to target, and their competition. You can use Google Keyword Planner(free) for this. Find out what people are searching for (search terms) on Google and Amazon, using their autocomplete feature.
