I would say browse through what's on the rack and decide which are half way decent and which are pure garbage. Barnes & Noble is nice since there's a wide selection and you can sit down. Most gun rags are fluff and glowing reviews since they don't say anything bad about advertisers.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
US Army 1966-69, VFW Life Member, Retired Geek