Buy a lower receiver first- Bushmaster, Armalite, Rock River, DPMS, Olympic.... Ordering the lower seperately means that you'll gain the benifit of not paying excise tax on the entire complete rifle.

When you decide what typeof upper you want, order the the upper. There are a myriad of possibilities- you need to determine what is right for your purposes, or even buy a few different uppers for differetn applications. Its really easy to install an upper onto a lower- just mate the two assemblies up and push two pins through.