Who was Joshua:

Joshua is first mentioned in Exodus 17:8 when he is called by Moses to lead the battle against Amalek:

The Amalekites came and attacked the Israelites at Rephidim. Moses said to Joshua, “Choose some of our men and go out to fight the Amalekites. Tomorrow I will stand on top of the hill with the staff of God in my hands.”

Exodus 17:8-9

According to Numbers 11:28 Joshua is Moses’ assistant:

Joshua son of Nun, who had been Moses’ aide since youth, spoke up and said, “Moses, my lord, stop them!”

Moses’ punishment for not giving God credit when drawing water from a stone (Numbers 20:12) was forbidding Moses and Aaron from entering the Holy Land. Numbers 27:18 reveals Joshua was to lead the Israelites into the land of Canaan following the death of Moses and assume Moses’ position as the leader of the Israelites.

Then the LORD said to Moses, “Go up this mountain in the Abarim range and see the land I have given the Israelites. After you have seen it, you too will be gathered to your people, as your brother Aaron was, for when the community rebelled at the waters in the Desert Zin, both of you disobeyed by command to honor me as holy before their eyes.” (These were the waters of Meribah Kadesh, in the Desert Zin.)Moses said to the LORD, “May the LORD, the God of the spirits of all mankind, appoint a man over this community to go out and come in before them, one who will lead them out and bring them in, so the LORD’s people will not be like sheep without a shepherd.”

So the LORD said to Moses, “Take Joshua son of Nun, a man in whom is the spirit, and lay your hand on him. Have him stand before Eleazar the priest and the entire assembly and commission him in their presence. Give him some of your authority so the whole Israelite community will obey him.

Numbers 27:12-20