You would need an add-on for global sticky threads/posts - check the Resources section for this.
Depending on what you want to 'stick', it may be possible for you to achieve this without any add-ons. You could use the built-in Notices feature in the ACP and get it to show in all forums. If the content of the 'sticky' isn't too big, that could just be typed into the Notice itself. If the content is too long, you could put some brief text into the Notice and add the URL for a thread where you would put the full details.