I'm not quite sure I understand exactly what you are asking, but I will try to help you out as best I can.
Yes, the upper needs to be .223, or 5.56, or what is called a "Wylde" chamber. For the differences on these, you can do a search for more details but a short answer is as follows: 5.56 is NATO/Military ammo and is loaded to higher pressures which benefits from the slightly more generous chamber on a 5.56 upper. A 5.56 Upper will shoot .223 but is generally thought to be slightly less accurate due to the larger chamber. A .223 upper will most likely not cycle 5.56 NATO loads reliably, but may have the potential to be more accurate due to tighter chamber tolerances. The Wylde chamber, and I have no experience with these, is supposedly a compromise between the two and will fire and cycle 5.56 or .223.
If you have no experience building rifles, or AR's specifically, and you don't want to learn, you can buy one complete. Might not be a bad idea for your first AR. You can learn about them, then build your next one.
Regarding the lower, same advice as far as building vs. buying a complete lower. The lower needs to be for an AR-15 type rifle.
Hope that helps.