everytime that I open the craigslist.org page ads appear on top and bottom of the page. What can I do to get rid of them?

Use Adblock extension in your Safari.

From the Safari menu bar, select

Safari ▹ Preferences ▹ Extensions

Turn all extensions OFF and test. If the problem is resolved, turn extensions back ON and then uninstall them one or a few at a time until you find the culprit.

If you wish, you may be able to salvage the malfunctioning extension by reinstalling it. That will revert its settings to the defaults.