How did the first king become king?

The first King of England was Aethelstan who was the grandson of Alfred the Great. In 924/5 Aethelstan became king of Wessex (or king of the west saxons) after the death of his father Edward, upon the death of AEthelflaed (Aethelstan's aunt) Aethelstan assumed control over the kingdom of Mercia. This united all the kingdoms south of the river Humber under one King.

Aethelstan married his sister to a viking king of Northumbria (north of the river humber) in 927 the viking king died and Aethelstan avanced his army into Northumbria and captured the city of York. In 937 the Battle of Brunanburh took place between Aethelstan with his brother Edmund against the Scots and the Irish vikings in which Aethelstan was victorious, resulting in him becoming king of Northumbria and thus uniting it with the south creating England. Aethelstan's reign over a united England was short as he died in 939.