The word "tithing" appears only once in the King James Bible (Deuteronomy 26:12), while the word "tithe" (from the Hebrew word for tenth, Strong's Concordance #H4643) and its plural occurs 38 times.
First Tithe
The first place many people believe a tithe is mentioned in Scripture is in Genesis. It occurs when Abram (later Abraham) gives the priest Melchizedek a tenth of the spoils he won after he battled several kings.
And Melchizedek the King of Salem brought forth bread and wine. And He was the Priest of the Most High God.
And He blessed him, and said, "Blessed be Abram of the Most High God, possessor of heaven and earth. And blessed be the Most High God, Who has delivered your enemies into your hand." And he gave Him tithes of all. (Genesis 14:18 - 20, HBFV).
God commanded the implementation of a tithe system of giving based on a person's "increase" (Deuteronomy 14:22). This was done in order to provide support for the tribe of Levi (Numbers 18, Deuteronomy 18).
The Levities were dedicated by the Eternal (Numbers 8) to serve the religious needs of the entire nation (e.g. serve in and maintain the portable tabernacle, which later became permanent in the form of Jerusalem's temple). Their support through a special tithe was needed since the Levities did not receive an inheritance of land (Numbers 18:20 - 21, Joshua 18:7, etc.).
Church Practices
Churches and fellowships vary widely on their views and practices regarding this teaching. Some believe that although the practice was commanded in the Old Testament (Leviticus 27:30 - 33, Numbers 18:20 - 32, Deuteronomy 12:5 - 6, 14:22 - 23, etc.) it was considered no longer valid in the New Testament. Others argue that the practice was not abrogated in the New Testament church.
Based On What?
Some that believe a tithe should be based on a person's gross income (before taxes and other deductions) while others state that a tenth should be given based on one's net income (income after taxes, expenses related to a person's employment or business, etc.).
Concerning the implementation of this doctrine, some churches leave it up to the conscience of individual members to determine what is their "increase." Other organizations, however, dictate to their membership when and on what they should tithe.
It should also be noted that the Old Testament system of tithing was also used for more than supporting the Levitical priesthood. A second tithe was to be saved and personally kept to pay expenses (travel, food, lodging, etc.) for a person and their family to observe God's annual Feast Days (especially the Feast of Tabernacles - see Deuteronomy 14:22 - 27).
A Third Tithe?
A possible third tithe existed in Israel for the support of the poor. It was to be saved every third and sixth year in a seven year cycle, a fact confirmed by the Jewish historian Josephus (Antiquities of the Jews, Book 4, Chapter 8, Section 22).
At the end of three years you shall bring forth all the tithe of your increase the same year, and shall lay it up inside your gates.
And the Levite, because he has no part nor inheritance with you, and the stranger, and the fatherless, and the widow, who are inside your gates, shall come, and shall eat and be satisfied so that the Lord your God may bless you in all the work of your hand which you do. (Deuteronomy 14:28 - 29, HBFV).
This third tithe was given to "the Levite, the stranger, the fatherless, and the widow" (Deuteronomy 26:12).