You can join an affiliate network as a merchant or advertiser. Each has their own requirements and other factors for consideration. You can refer to your favorite search to find one that fits your needs.
That's a general question that is way too complicated to cover here in one reply.
Here are the *basics*:
1. First you need to create your affiliate program. You'll need to determine how much per sale you want to offer, when you are going to make payouts, etc. You'll also need to create affiliate content and graphics in the most commonly used sizes that your future affiliates can use on their websites.
2. Since you are just starting out check out ShareASale.com. They will help you with all of #1 and offer a bunch of info and tools to help you setup a successful program.
3. Then, create a landing page for Affiliates on your website. Explain why they should promote your products, the details of $ per sale and the support and tools you offer to help them help you. You would then link to your ShareASale sign-up page so they can join the program.
While the above is just 3 steps, there is a ton of planning and work that you need to do to get everything in place before you start courting potential affiliates on your website. Doing a search at your favorite search engine for "how to start an affiliate program" will provide numerous articles for you to do your due diligence.