The Bible asks us to give our first and our best back to God, and He will bless the rest (Prov 3.9-10). 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 90-Day Tithe Challenge. Essentially, it's a contract based on the promises of God in Malachi 3.10.
