If you're a noob I wouldn't suggest trying to build one. Like custom crafted cars, they can be cantankerous and need a lot of tuning, fiddling until they're where you want it to be. I'd start out with the S&W M&P 15 Sport just to get familiar with the AR platform; the sweet part about that gun is that it's about $700, more or less. I'm pretty sure it's chambered for 5.56 so you can also use 2.23 if you like. The bullet button is already installed, you get a CA-capacity 'mag, so legality wise you're good to go. Now if you believe in buying the best that you can for the little money you've got I recommend going with the Colt 6920. It's about $1200 but you're getting Colt performance and reliability for a little over a grand. If you decide you want a fancier 'AR or a gas piston one in the future you can always sell your Colt for almost what you paid for it. Buy once, cry once, as some here would say, and either of those guns will serve you well. Look around the intrawebz for reviews/opinions about either gun and you'd be hard pressed to find anything negative about them...
From days of long ago, from uncharted regions of the universe, comes a legend; the legend of Citadelgrad87, Defender of the Universe, a mighty robot, loved by good, feared by evil.