Originally Posted by pawjr74
I would concentrate on sights first. Did it come with sights? Decide if you want iron sights or an optic. Next would would be the forward grip. Do you like it the way it is now or do you want a vertical or angled front grip? Shooting it may answer some of these questions for you too.
I agree with pawjr74
but I also agree with the folks who say (sometimes not very tactfully) you should shoot your gun enough - at least a few boxes of ammo if you can get it - to get a feel for it and see what feels "right" and what doesn't. Then
, start changing the things that don't meet your expectations.
As to your original question, the most common modifications you'll find, especially with newbs, are to sights, stocks and foregrips. These are also the easiest items to change/upgrade for someone new to the AR platform. Once you've had it for a while and know more about your gun and AR's in general, you can decide what other "improvements" you may want. BTW, a lot of the things you may want to consider aren't as much upgrades as accessories like extra mags, carry bag, slings, optics, etc. that can be used with other rifles and/or transferred to a new gun should you decide to replace you current rifle.
I personally don't own a Carbon 15, but do have several M&P-15's and a Bushmaster XM-15. OTOH, I know people who do own a Carbon 15. They all seem happy with the gun and say it does what they want it to do. It may not be a higher end Ruger, NIT, Knights or Colt but they also don't cost $2,000-$3,000+ dollars either.