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.
Be sure to check what kind of customer support you can expect from your affiliate program once you have signed up. Do your research online and if possible, speak to other sellers using the program to get their thoughts. Can you speak to someone via phone or Skype or do you have to wait 72 hours for email responses? Be clear on this because trust me, you will need support at one point or another.