Hi, I own a Core 15 M4 that was $826 plus tax and $50 for a rear sight.
There are a lot of good AR's on the market. First you should look at the BCG to see if the gas key is staked properly, it should have little indentations on the bolts.
Second you want a 5.56 NATO barrel, preferably chrome lined as it is more durable than steel but generally a little less accurate, though most shooters won't see a difference.
A 1:7 twist rate is what most people want. My rifle has a 1:9 twist rate but 1:7,1:8 and 1:9 are optional. The 1:7 twist allows you to shoot heavier grain rounds more accurately.
Core, Spike Tactical, Bushmaster, Colt, S&W all make good AR's.
The M&P-15 is a good beginner AR, although it lack some features such as a dusk cover, forward assist, and chrome lined barrel. Also can't except those 60+ round mag.
For the price Core and Spike Tactical come to mind for all those standard AR features and are $800.
Theres more I want to say but no characters, Try Youtube
Type in good first AR-15's, or they Google. Also I reccomend carbine length unless you plan to do long distance shooting/competition.
Here a line to a previous AR question I got BA for, maybe it can help.