my tip would be Decide what rifle you want to end up with, Pick each part write it down or go on AR builder.com then order that, not buy a basic one and spent your next 6 months wages on little bits and pieces. Im going to stick my neck out and say what makes a ar15 accurate is the barrel and trigger. barrel must be of good quality and be free floated. trigger must be easy to use. good mounts and good glass come next.
BCG - bolt carrier group. this is the bolt of the rifle
Free float tube - this is the fore grip of the rifle by free floating it dosent contact the barrel aiding accuracy
lower - the bottom bit that holds the trigger, stock, and magizine well.
upper - the top bit that has the barrel, fore-end, sights, gas tube and houses the charging handle and BCG
If you suppress your AR15 and start getting debris and gas blown into your eyes do this -
Do It Yourself Gas Busting Charging Handle [2009-09-29] - 03DESIGNGROUP
I disagree about the cleaning comments above, barrels must be looked after as much as any other rifle. inside and out. chrome lined I have no experiance with.
get a crome lined BCG if it is offered they are great.
the rest of the rifle is basically Ali. bar the pins trigger and springs ect.
Go light on any oil you spray into the action area as it can cause extra burning of eyes