As simple as it sounds, you need to find where your particular affiliates hang out. Those that will be the best fit to promote your product or service. Then figure out how to reach out to them. Usually direct, polite contact is best.
Also, know that it takes time to find good affiliates. Probably 90% of affiliates won't do much if anything. Not being negative, just realistic. I have a bunch of affiliates for products I have on JVzoo, but few make any sales.
Friend of my took his time to track down the right affiliates for one of his new products. Ended up with only one good one, but it netted 10k in sales. So it's worth doing the research and doing your homework.
elcidofaguy had some great ideas. Plenty of places to announce your program. And yes, for sure you HAVE TO HAVE promotional materials. It amazes me how many programs, like those on clickbank for example, that have like zero promo materials. I skip them and I am sure many others do as well.