Even though he's been hanging around helping Moses since Exodus, Numbers is really Joshua's time to shine. Hey, someone's gotta lead these people into the Promised Land after Moses kicks the bucket.
Joshua is described as "the assistant of Moses" (11:28), but instead of fetching him lattes all day, Joshua was probably more like a trusted friend and advisor. In fact, when he sees a bunch of guys prophesying around the camp, Joshua runs and tells the boss right away. Moses is cool with it, but his #2's loyalty credentials are cemented.
He's also one of the spies Moses sends out on the scouting mission. Like Caleb, Joshua thinks that Israel can totally take those Canaanite giants. That one little principled stand earns Joshua a ticket into the Promised Land (26:65).
Later, Moses appoints Joshua the new leader of the community:
So the Lord said to Moses, "Take Joshua son of Nun, a man in whom is the spirit, and lay your hand upon him; have him stand before Eleazar the priest and all the congregation, and commission him in their sight. You shall give him some of your authority, so that all the congregation of the Israelites may obey. But he shall stand before Eleazar the priest, who shall inquire for him by the decision of the Urim before the Lord; at his word they shall go out, and at his word they shall come in, both he and all the Israelites with him, the whole congregation." (27:18-21)
Joshua's next big move in the Bible comes in—surprise, surprise—The Book of Joshua. There he parts the River Jordan, wins the Battle of Jericho, and finally (finally!) leads the Israelites into the Promised Land.
All in a day's work for the newly appointed leader of Israel.