I think the only 2 things you "need" to upgrade/improve the function of the rifle are the gunfighter charging handle (don't have to worry about broken latches anymore) and extractor oring (improves extraction). Other than that, your rifle is fine, for NOW, until you figure out what else you want to change. Everything thing else in BRD (Black Rifle Disease) is personal preference and completely up to the shooter.

I also echo ammo, mags, and training.

I'm assuming you got a magazine with your rifle and probaby bought a box or two of ammo.  Not knowing what kind of shooting you do (sorry if it was in the last two pages but I got tired of the Ninja advise).  A trigger is probaby going to help your accuracy the most.  I would suggest a drop in of some kind.  I have a Timney with 4 lb pull and love it.  3 lbs is just too light for the kind of shooting I do.

How could this NOT have been the very first suggestion?  The trigger makes THE most difference in accuracy.  Even a Bill Springfield $35 trigger job will improve the accuracy HUGELY!  Personally, my favorite trigger is the AR-Gold.

Just bought a new S&W MP 15 what would you upgrade?

nuttin', I like mine outta de box.  

THIS!!     Shoot the hell out of it, then decide what you need or want!

Isn't the OP asking for advice based upon our collective experience? 'Do nothing' is one man's opinion based upon your and other's experiences. My, seemingly extensive list, is just what I add to each new AR in the arsenal as they come in. That's based upon my experiences and which I'm sharing, as requested, with the OP. I'm not disparaging your keep it bone stock, try to find ammo (likely a bad idea as cheap, dirty burning, ammo is the last thing the OP needs in his new EBR) advice.

Don't jump on my case for having a, well tested, set of opinions and time-tested standard of upgrades for my specific purposes. My weapons don't sit on a display stand by the TV. They get shot, a lot.

Besides mags and ammo, a sling, light and optic. The rest can wait until you've used it a bit. It's definitely fun to bolt on 20 pounds of crap onto your rifle. But after humping it a while or using it, your rifle will shed weight realllllllly fast.

I would buy a technical manual , magazines and ammo . sights if not installed or an optic. I have a m & p 15 and It will get the job done as it sets right out of the box .  there is not an immediate need to swap or change any parts.

This..

Sling for functional carrying, upgrade number of mags/ ammo, and purchase some decent targets if your ranges are anything like what I have up here in ND.. Set up your own, open to the public, we have rows of earth and tires covered in rubber to hold the targets you bring.  That said.. 10-12" gong steel targets are very fun after everything's broken in and zeroed.

Only thing that I changed on my rifle within the first 1000 rounds was added a new end plate to the upper receiver for the attachment of a 1 point sling.

Not sure it's really an upgrade or not, but I put an H buffer in my M&P 15 to an effort to slow down wear and tear from being over gassed.  The recoil softened up a bit when shooting M193 and M855.  Probably could have gone with an H2.

I consider the Magpul trigger guard, MOE + grip, MOE handguard, and VFG I put on to be upgrades from the original furniture.