I am building my first AR and was wondering what to watch for, as far as what is a pain, what is very critical and just some general hints for a first time builder.

Use masking tape to tape up the lower to avoid scratching it up.  Install the pivot pin detent and spring in a trash bag or large ziploc bag to avoid sending the pin into orbit should it (and it will) pop out.

Chase out all holes and passages before putting parts in.  Having a detent pin stuck half way into it's bore is a bitch.

Hoppy

For the front takedown pin: press the detent into its hole using the end of the takedown pin, then rotate the takedown pin into the receiver "ears".

Painless, and you don't need a tool

"Patience is one of the fruits of the Spirit."

Go slow.  Don't force anything.  Nothing goes in with any big smacks.  Fire group pins can almost be put in with strong thumb pressure.  I have an extra set of fire control pins that I chucked in a drill and buffed down with some 600 grit emery cloth.  I can set every thing up on these dummy pins first and then push the real pins with a gentle tap, displacing the dummy pins.  some people use nails I hear. The only pins that need to be driven in are the roll pins and they should still go in with gentle taps.

Get some 600 grit emery cloth and go over the leading edge of all your pins to insure easy starting and insertion.

Use Moly lube or some other moly grease liberaly during assembly.  Don't assemble it dry.

Pre test parts for fit first.  Dont push the magazine catch/release spring and push pin in with out a test fit first. It should go in and out easily.  I've heard more than one horror story where some one has pushed it in with their thumb and the damn thing is stuck and requires drilling from the inside the lower to get it out.  Chase the hole first and buff the pin with that 600 grit.  Lube judiciously.

Use thick leather to hold all parts in your vise.  It probably won't affect function but it will look much better when you're done.

Ask questions here as you go along.  You may have to wait 20 or 30 seconds for someone to reply.  They may be right....they may be wrong.

If they're wrong it will take about 2 seconds for some one to point it out and jump in the imposters shit.

Again..."Patience is one of the fruits of the Spirit".

I'm a newbie also but have built a couple rifles.  Duct tape works well to protect the receiver during assembly which might scratch it.  Also, I recommend the use of a 1/4 inch clevis bolt(75 cents at HD) instead of a tool to install front pin.  Buy one andd it should be evident how to use it.  Work slow and deliberate.  The information on this site was very helpful.


Rick  

Also, point the Detent towards a opened plastic bag(WOrk inside the bag) so if it flies out, the bag catch's it.

The one really big "thing" I learned about building ARs was, nothing needs to be forced. Even the roll pins are installed with very light taps.
Don't be in a hurry, and when you have to take something apart becuase you forgot something, watch for flying detents.
When you first get your lower, look it over closely. You will notice several small holes. Something goes in everyone of them.
If you decide to build your upper, don't trip about putting it together. Touqueing the barrel
is a piece of cake.
Do a bunch of searchs and all the questions you could possibly have most likely have been asked at least once.  

The front detent pin is the first real booger, and it's been covered already.

The second is the winter trigger guard roll pin.

My buddy found out the hard way that if you just try to tap it in, and it wedges, and you keep hitting it harder and harder, eventually one of those two fragile "ears" will break off, pretty much ruining your receiver.

A better way, which has been mentioned here, is to SUPPORT the bottom ear (when tapping the roll pin on one side, with the receiver laying down on its side) until you've got the pin STARTED...

And, remember, you've taped your receiver here so the jaws of the vise-grips you're about to use do NOT contact the metal...

Use a large set of Vise-Grips to gently SQUEEZE that triggerguard pin into place, keep it straight.

Either that, or you can slowly push the pin in using a vise itself, and a couple of wooden blocks.

THAT was the biggest booger that ruined a receiver in my experience.  I was there when my buddy ruined his...and I was supposed to be the one with the experience...but I'd never experienced that before.

Learn from our oops.

Panz
[bounce]

Well now that my lower is complete I now see that I need to take it apart and lube some things up.  Now where "specifically" do I need to use the moly lube?  The one place I looked at and said "I need something on these thread" is the buffer tube.  Thanks for the help