I looked at the info on the Build it Yourself sticky's at AR15.com.
I also started with a good parts list. On a first build, it might be best to get:
Lower parts kit
Buttstock assy with buffer, tube & spring
Complete Upper Assy
Bolt Carrier Group
Sights ... Some parts maybe in upper assy
Sling and sling mount
Optic and mount
PSA is a good place to look as well as BCM.