AR15 Barrel Profiles
AR-15 Gas System Lengths
Upper Variants & Anatomy
Those should give you a few good primers. One I couldn't find a decent article on was rifling twists. Be aware that AR-15's are sold with a variety of rifling twists, and this will have an impact on what type of bullets you'll be able to shoot (with any degree of accuracy). Long story short, if you think you'll want to shoot any of the 69gr or heavier bullets, go with a 1:8 or 1:7 twist barrel.
There is a LOT of info out there. There's also a lot of unnecessary accessories, biased brand loyalty, and opinions of "must haves" that aren't based in reality. Remember in shopping around that there's a fine line between buying affordable and buying junk; there's just as fine of a line between buying quality and buying a name/gimmick. Ask questions here, there's plenty of folk willing to help. When in doubt, go with what Molon says.
Rather than getting overwhelmed with everything that's out there, outline the features YOU want in a rifle and try to find one that fits that criteria.
Building one is a good option, I've done it myself and helped others. Just be aware that the boxes of parts you end up with may not all mesh if you didn't do your due diligence before adding parts to your cart and clicking "buy." It's then that you find out things like your gas block wont work with your handguard, your stock wont fit the receiver extension, the upper has M4 feed ramps but your barrel doesn't etc, etc, etc.
Hope this helps, good luck!