What would I need to create a home alarm system

The decision of which security system to get is very personal, and depends on many different factors, including budget, technical requirements, maintenance requirements, type of events you want to track, whether you want professional monitoring, etc. There’s no one right answer.

At this time, there’s no easy way to integrate SmartThings with any security system. It’s not impossible, but it requires considerable technical expertise. So that’s probably not going to be a consideration in what you select.

There are many topics in the forums discussing how different members have approach security issues, so you might just search the Project Reports section of the quick browse lists in the community-created wiki for security and see what you find.

http://thingsthataresmart.wiki/index.php?title=How_to_Quick_Browse_the_Community-Created_SmartApps_Forum_Section#Quick_Browse_Links_for_Project_Reports.2FQuestions

Probably the easiest is the cloud to cloud integration with Simplisafe. This is a solid inexpensive security system. Integration with SmartThings only requires installing one custom smart app, and you will be able to see information from your simplisafe devices and use that information to trigger Smart
Things events. It won’t be the right answer for everyone, but as a simple answer for use with SmartThings, it’s probably the most developed right now.