How do I set my siteURL instead of having http://localhost:8888/?
Unless instructed otherwise, MAMP uses port 8888. You can tell it to use the "default" ports, which will mean the site gets served over port 80, and your URLs no longer contain :8888.

As for the other issue, it sounds like you haven't set up your site's document root correctly. Either way, it's a MAMP configuration issue, not a Craft issue.