God instructed in the Old Testament to give a tithe, or a tenth because this ten percent represented the first, most important portion of all they had.
The New Testament does not suggest a certain percentage for giving, but simply says for each to give "in keeping with his income."
God wants us to give because he knows we will be blessed when we give generously to him and to others.
Giving is a paradoxical kingdom principle it brings more blessing to the giver than to the recipient.