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.
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.