My posts (not comments) are getting spammed

Hello Everyone

Even though I’m no good at PHP and only understand some basics, I managed to put together my own theme. Everything works, but I’m afraid I’ve made a huge mistake somewhere, because my posts are getting edited by spammers. They write right into my content and place links to random websites. Can someone point me in the right direction here? What do I need to check / fix in order to stop this from happening?

Thanks in advance 🙂