Take your time, read massive amounts of information on the AR/15 platform. There are many forums dedicated tot he AR/15?M4 platform and many configurations that may be of your desire. Here are some tips:
1. what do you want this AR to do for you ? Long range, short range, both or a comprimise between the two ?
2. What configuation ? M4 ? M16 ? SPR ? What are goals with this rifle and how does it fill your needs overall ?
3. Mill Spec or National Match type configuration ? Do you want this rifle to just plink ammo or do you want it to put out sub MOA at range or both if possible ?
4. Home defense ? Maybe not, but a shorty with light frangible ammo might the ticket if your stuck on stupid.
5. What grade of AR do you desire ? Mil Spec, commercial, match grade, varmint etc ?
6. What type of dope are you expecting to use ??? Just anything that's in the Wal Mart shelves or hand loads with specific match grade ammunition ?
7. Or, are you just another AR fanatic that wants whatever is on the shelf in the local FFL that fits your budget ???