If you are looking to start guest posting on other sites, you should simply reach out to other web masters or bloggers in your niche. Perhaps with an email or through social media. Talk to them about collaborating on a post or offer them something in exchange for letting you post on their website.
If you want people to guest post on your site, reverse your tactics in that you should offer them something to post on your website (assuming you are just starting out). If you already have a lot of traffic and notoriety they may do it for free as long as you let them put links and such of their choice in the post.
Here are some guest posting tips that might be of use,
Hope that answers your question,