Are you asking as a marketer? (sell other peoples product) or as a company wishing to create an affiliate program?

If it’s the first, then it all depends on the company you want to sell products for. Each company with affiliate program either use an affiliate management company or create their own in house.

If it’s the latter, then you need to look at incorporating an affiliate program,install it in your site, create the banners and links, and sometimes manage it. Starting an affiliate program is much like having distributors, you need to keep them up to date, motivate them to keep on selling, giving them tips etc.

With this you have two choices:
1. Hire affiliate management company for you to do the whole thing, including paying the affiliates. They will charge a set up fee, plus ongoing fees such as SHAREASALE, COMMISSION JUNCTION.
2. Hire a freelance ( tech savvy moms) to find the best software (such as http://www.idevdirect.com, jrox.com), set it up, and you do the managing and paying the affiliates their commissions.