The word "tithe" simply means "a tenth" -- and it refers to the Old Testament's teaching that we should give back to God at least a tenth of our income.
Why is this? One reason is very practical: God's work requires money -- just like every other enterprise. Even Jesus' little band of 12 disciples had a treasurer, and we read that a number of people not only followed Jesus but supported His ministry with their wealth (see Luke 8:1-3). Many churches and ministries are facing hard times today because of the difficult economy.
But the Bible teaches us another important truth that's easy to overlook: All that we have -- not only our money, but everything else -- comes from God. We think that because we worked for it, our money is ours, and we can use it any way we want to. But that is wrong. Everything -- including our ability to work, and everything else -- comes from God's gracious hand. Why shouldn't we give some of it back to Him? Why shouldn't we seek His will about everything that remains?
As a footnote, don't be embarrassed to ask questions of your pastor or of others who can help you. God loves you, and He wants you to grow in your faith. Commit your life to Christ, and then make it your goal to "grow in the grace and knowledge of our Lord and Savior Jesus Christ" (2 Peter 3:18).