In order to give a good answer, we need a little more info about how your rifle is set up and it's intended uses. There are enough options that you will get a headache, so be prepared.
Are you wanting to attach a 2 point tactical sling, a simple carry sling or a single point sling?
Which particular model of M&P 15 do you have? A model number from the box label would help tremendously. If you don't have that, a photo of the front end of your rifle, including the front of the foreend and gas block would help.
Unfortunately for some of us, getting a sling set up the way we want is almost as hard as choosing a holster for a carry gun. Most of us have gone through several setups before we finally hit on the right combo for US.