Do you mean a round neckline or an actual pointy collar?
If you mean a round neckline you need to add some ribbing or Fold-over Elastic google FOE and you'll find what I mean.
is a shirt I made and did the cuffs and neck with FOE. This one is gathered at the neck but if you don't stretch it much as you sew it on it will just hold the neck flat and not get saggy.
You can use strips of t-shirt to bind the neckline too. Cut strips across the shirt about and inch or so wide then fold them in half lengthwise and sew onto neck right sides together stretching the strip just a little bit.
For an actual collar you need some interfacing to stiffen it up a bit.