Many affiliate programs are free to join. This is the ideal so that even if the benefits and compensation are limited at the free level, you are able to see what the program is about in depth. A free member should be able to earn commissions for their sales, albeit maybe at less than an upgraded member (not always possible, up to the program).
The best affiliate marketing program will have some or ideally all of the things you need to start up a business from home. Most come with a free web page to advertise and an affiliate link so they can identify your commissions.
If you can, you may want to join a few programs and compare them and then upgrade the best one and focus only on that. The smartest way to go is to have multiple streams of income so that you will have many opportunities to earn money. Whatever works best for you, this is the time to find out.
The best affiliate program will have good support and training. There will be a FAQ that explains most of the common questions for your information, as well as to help you talk to your prospects, articulate ads, etc.. There should be at least one way to contact them and get a response in a reasonable amount of time. Test this as soon as you can.
Better quality programs will also include training on some level to help you get started. This might be general information about marketing online or may be specific to the product that you will be promoting. This training should be accessible to the members 24/7 so that they can work at their own pace.
The best affiliate program may include a way for you to network with your peers. It may be in a forum, chat group or webinar environment. This is fairly important as you can help each other which is an important benefit. You can also get new ideas and advice from people who have had success.
Top of the line value would include management that is accessible. While dealing with 1000 people on a personal level is impossible, if they have some sort of a group meeting online monthly it would be a big encouragement to newbies. A ‘current event’ newsletter for the membership would also demonstrate the owners are actively involved and are there to help.