The Bible asks us to give our first and our best back to God, and He will bless the rest. We return to the Lord 10% of what He's given us because it belongs to Him.
If you're not tithing already, a great way to start is with the Three-Month Tithe Challenge. Essentially, it's a contract based on the promises of God in Malachi 3:10-11.
Test me in this and see if I will not throw open the floodgates of heaven and pour out so much blessing that there will not be enough room to store it. - Malachi 3:10