I'd say enjoy the process of deciding on what to get and the process of getting the parts together. For me, it's almost as much fun as actually shooting.
I've got too many built to start a new one just now (although I'm hankering for an SBR and that might be future project), but the starting point for me would be to start shopping around for a lower, a lower parts kit, and a stock. There are starting to be lots of good deals on lower receivers (many for less than $100).
While you're shopping for a lower, spend a lot of time looking at others' builds and trying to decide on components. ar15.com is a good place to see lots of photos.
There are lots of choices for stocks. Some are really expensive and some are relatively cheap.
Once you've got the lower components together you can start thinking about how you want your upper to be configured. Personally, I would build my upper. You might consider going ahead and getting an upper when you buy your lower so that they will match.
At some point you'll have to decide what type of hand guard you want (free float or conventional). Free float hand guards are great because they have picatinny rails that allow you to add accessories. The downside is that they're usually a lot more weight.
When shopping for a barrel, you'll need to make some decisions. Pick a twist that is compatible with the bullets you want to shoot. 1/7 twist is good for heavier bullets, 1/12 for lighter bullets. A good solution is to pick something in between (like 1/9). A lot of builders (myself included) want a chrome lined barrel, because it is easier to clean and holds up better.
You'll also need a flash hider, sites, and a sling. Be prepared to spend more than you expect. Also read and study a lot as you're deciding. There are a bunch of internet resources and you'll learn a lot as you progress.
Most of all go slow and enjoy the process of building a rifle exactly like you want. Enjoy your build!