In addition, if you choose to display prices for any Product on your Site in any “comparison” format (including through the use of any price-comparison tool or engine) together with prices for the same or similar products offered through any web site or other means other than an Amazon Site, you must display both the lowest “new” price and, if we provide it to you, the lowest “used” price at which the Product is available on the Amazon Site.
When do you want to get started?  If you are just starting and have no audience then some of the programs may not accept your application to become an affiliate, while others will let you get set up in minutes.  If you want to get started straight away then make sure you’re applying for programs that are less restrictive. Generally the larger, network style places will enable you to get going quickly while specific niche programs may require you to have a certain audience size or characteristics.
Who is your audience? What is your target market or niche? If you're targeting a specific niche like home security then perhaps you only need to sign up to ADT and SpyBase so your products match your audience. There’s no point promoting eco-mattresses to your blog that’s focused on reviewing drones. Again, you could sign up to a network that has a few different options in your field of interest or just go straight to your favourite suppliers and see if they have an affiliate program.
The topic you choose must have enough depth that you can create a lot of content for it. This is important for building an authoritative site, for search engine optimization, and most importantly, for the end user. If you don't have enough content about a topic, you're not going to be taken very seriously as an authority on the topic and it's unlikely you can convince someone to make a purchase from you.