This is a good question. The twelve disciples are the people that Christ chose to be with him.
Here is a typical account of Matthew, Mark and Luke:
And when day came, he called his disciples and chose from them twelve, whom he named apostles: Simon, whom he named Peter, and Andrew his brother, and James and John, and Philip, and Bartholomew, and Matthew, and Thomas, and James the son of Alphaeus, and Simon who was called the Zealot, and Judas the son of James, and Judas Iscariot, who became a traitor.
The kingdom of God is something different. It is the rule and reign of God. For example, when Jesus heals people, the power of the kingdom is present. Similarly when Jesus drives out demons, the kingdom is present as well.