The Wall is raised, and the Watch takes its vow
Bran the Builder ÔÇö the same name the songs attach to Winterfell and to Storm's End, which should tell you how much weight it can carry ÔÇö is said to have raised seven hundred feet of ice across the neck of the continent, and the Night's Watch was founded to hold it. The order kept the vow for eight thousand years, long after the realm stopped believing there was anything to keep it against.
The fuller tale
When the dark lifted, somebody built a wall. Seven hundred feet of ice across three hundred miles of the continent's narrowest point, and eight thousand years later it was still standing and still the largest structure made by men anywhere in the world. The songs give it to Bran the Builder, and give him Winterfell and Storm's End and a hand in the Hightower besides, which is a great deal of work for one lifetime and tells you what kind of name Bran the Builder is.
The maesters who have looked at it closely are careful to say that the Wall is not simply piled ice. Something holds it. Whatever that something is, it is old, it is not written down anywhere, and the order sworn to hold the Wall stopped being able to explain it a very long time before the end.
The Night's Watch was founded to man it, and took a vow with no term in it: no crown, no lands, no wife, no children, and no leaving. That vow was then kept, more or less, for eighty centuries. There is nothing else in this chronicle that lasted a tenth as long.
The people on the wrong side
A wall drawn across a continent decides who is inside it. The men left north of the ice were the same people as the men left south of it ÔÇö same blood, same gods, same hard country ÔÇö and within a few generations the two halves had stopped recognising each other. The ones inside called the ones outside wildlings. The ones outside called themselves free folk and have declined to kneel for eight thousand years, which is a long time to keep making the same point.
The Gift was granted later, a band of land south of the Wall to feed the garrison, and doubled centuries after that by a queen who visited and was appalled by what she found. By the last century both halves of it were mostly empty, because a farm within a raider's ride of the Wall is not a farm anyone keeps.
Elsewhere in the wiki
 See the world as it stood in 7,980 BC What became of it  if an ending is written, it is here. Spoilers.
The chronicle records no ending here ÔÇö or none the maesters have set down.