Aaron is chosen . . . and the concept of a tithe is introduced

In chapter 17 of Numbers, each of the twelve tribes gathers a walking stick and the leader's name is written on it.  For the tribe of Levi, the Lord tells Moses to have Aaron's name written on theirs.  Then, the walking sticks are to be placed in God's Tent to see who is chosen.  He did this in an attempt to stop the constant complaining His people were doing.

The next day, the stick from Levi's tribe--Aaron's--is chosen, as it has budded and blossomed.

Chapter 18 describes the duties of the priests and the Levites--the latter have a responsibility to God's Tent and serving those with the sacrifices.

In this chapter we also see the word tithe.  Tithe means a tenth.  All of the other tribes must tithe to God, and those tithes are to be presented to the Levites to use.  The Levites, in turn, tithe their portion of the goods they've received as a special contribution to the Lord.

The tithe given to the Levites are for their service to the Lord.