When someone starts reading the Bible at Genesis and intends to read it straight through, he or she often does well at the beginning. The stories in Genesis and Exodus provide great drama and interesting discussion. However, many people get bogged down in the next several books - Leviticus, Numbers, Deuteronomy. These are the books of the Law (Genesis and Exodus are too, but they are more narrative). The Law of Moses contains commands and instructions for Jewish life in the Old Testament. It covers all things political, sociological, and religious.
Before we go any further in our discussion about tithing, we need to lay out a few important details about the Law of Moses in the Old Testament.
The Law is Holy
This is essential to understand. The Law came from a Holy God and is a reflection of His holiness and perfection. The Old Testament saints praised it (a cursory glance through Psalm 119 shows this clearly) and Jesus honored it, saying that he did not come to abolish the Law, but to fulfill it. Because we know that New Testament Christians are not bound by the Law, we sometimes treat it like it's irrelevant or not important. Not true. The Law is every bit the word of God as any other portion of Scripture.
The Law reveals man's innate sinfulness
The Law was essentially a written code that condemned man and proved he could never earn God's favor on his own. We all stand hopeless before the Law.
Although Holy, the Law is powerless
The Law could reveal man's sin, but could not do anything about it. Now, the Law provided for sacrifices to be made for sins, but these were messy (downright gross, in fact - involving words like entrails), costly, and temporary (the sacrifice had to be made every year).
The Law was intended to drive men to Christ
As man realized his imperfection, he recognized his need for a savior. The intent is for man to be driven to the point where he cries out with the Apostle Paul, "Wretched man that I am, who will save me from this body of death?" It is then that man is ready for the good news of the gospel.
Jesus met all the requirements of the Law on our behalf
In so doing, Jesus fulfilled the Law and took the punishment for our sin (that's what the cross is all about). Jesus' death on the cross was indeed messy and costly, but it was also permanent. The book of Hebrews has so much to say about this, but I recommend you check out these verses in particular: 9:11-14, 9:24-26, 9:28, and 10:19-22.
Through faith in Jesus' substitutionary sacrifice, we are freed from sin and any obligation to the Law
Jesus paid it all. There is nothing to be gained by following the Law of Moses. In fact, in the book of Galatians, Paul is astonished that the people would consider going back under the Law after having been set free by Christ. He goes so far as to say that the person who wishes to continue in the Law is cursed. Why would he make such a strong statement? Because he knows that the Law cannot save - it can only condemn. And the only way to be free from that condemnation is through faith in Christ.
Christ is the end of the Law (Romans 10:4)
The Law was nailed to the cross (Colossians 2:13-14)
Christians are not under the law, but under grace (Romans 6:14)
The Christian who seeks justification under the law has fallen from grace (Galatians 5:4)
Now the righteousness of God is revealed without the aid of the law (Romans 3:21-22)
This primer on the Law is intentionally short and woefully inadequate, but hopefully provides answers to rudimentary questions.