You can put up banners on your site, to promote your affiliate offers. Most affiliate programs will usually provide their own creatives when you sign up for their offers. All you have to do is insert the banner on a highly trafficked page (your affiliate tracking is usually embedded within the code). Banner ads in the right locations can do a great job of driving sales.
If you’re in the WordPress industry like I am (whether it be design, development, or SEO) I have accumulated quite the list of WordPress affiliate programs. I excluded those I found unsuccessful or pay too little to make a profit from, specifically ThemeForest, Creative Market, and low quality theme stores like Template Monster. Hosting pays well and I wrote a tutorial for SiteGround’s affiliate program and StudioPress themes which are my 2 highest paying affiliates. Those tutorials have tons of screenshots/social proof especially for SiteGround.
To become involved in affiliate marketing you should join a program – try ClickBank to begin with if you are new to affiliate marketing – then advertise products on your website or blog including your affiliate reference in the sales link. Your affiliate program explains how to do this, and most will provide graphics and promotional material to help you sell.