If you're interested in writing for a certain website, you should first check if they have a Submissions tab. If they do, check if it's open, and if they are, feel free to adhere to the guidelines and apply to submit your guest posts. If submissions aren't open, DON'T submit! Most websites and employers find it very annoying when their inboxes are full of amateur writers begging to be featured. Don't be one of those people.
If a website doesn't seem to have any submission sections, you can try to go to a contact tab and contact the editors, or the bloggers on the website. Be sure to address them nicely, and maybe attach a sample of your work. If they allow the submission - great! If not, accept the rejection graciously. Don't be a poor sport.