Tithing (the giving away of a tenth of our income to the local church) is a controversial issue with many.
Not hard to see why. It is, after all, OUR MONEY we're talking about!
The main issue with tithing is whether or not it is something Christians need to do now in these New Testament times.
In his book, Counterfeit Gods, Timothy Keller - Senior Pastor of Redeemer Presbyterian Church in New York City - gives this great answer:

There have been times when people have come to me as their pastor, and asked about ‘tithing,’ giving away a tenth of their annual income. They notice that in the Old Testament there are many clear commands that believers should give away 10 percent. But in the New Testament, specific, quantitative requirements for giving are less prominent. They often asked my, ‘You don’t think that now, in the New Testament, believers are absolutely required to give away ten percent, do you?’ I shake my head no, and they give a sigh of relief.
But then I quickly add, ‘I’ll tell you why you don’t see the tithing requirement laid out clearly in the New Testament. Think. Have we received more of God’s revelation, truth, and grace than the Old Testament believers, or less?’ Usually there is uncomfortable silence. ‘Are we more ‘debtors to grace’ than they were, or less? Did Jesus ‘tithe’ his life and blood to save us or did he give it all?’ Tithing is a minimum standard for Christian believers. We certainly wouldn’t want to be in a position of giving away less of our income than those who had so much less of an understanding of of what God did to save them.

Timothy Keller, Counterfeit Gods, Hodder and Stoughton, 2009, pg 62.