The first place many people believe tithing 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 (Genesis 14:18 - 20).
God commanded the implementation of a tithing system for ancient Israel, based on a person's "increase" (Deuteronomy 14:22), in order to provide support for the tribe of Levi (Numbers 18, Deuteronomy 18). This tribe was 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). This support through tithing was needed since the Levities did not receive an inheritance of land (Numbers 18:20 - 21, Joshua 18:7, etc.).
Modern tithing is the practice of a person giving a tenth of their "increase" to support a religious purpose such as preaching the gospel, support of paid pastors, church-related expenses, and so on.
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 it was considered no longer valid in the New Testament.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.
Some that believe in tithing teach that it 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" on which tithing is based. 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 possible third tithe in the tithing system existed in Israel for the support of the poor. It was to be saved every third and sixth year in a seven year cycle (Deuteronomy 14:28 - 29), a fact confirmed by the Jewish historian Josephus (Antiquities of the Jews, Book 4, Chapter 8, Section 22). This contribution was given to "the Levite, the stranger, the fatherless, and the widow" (Deuteronomy 26:12).