2 Corinthians 9:7
The above verse is the only "rule" about giving in the New Testament. We are told to tithe 10% of our income. And that is a good place to start, but that is a manmade rule, not God's for New Testament life. God wants us to give cheerfully. It all belongs to Him anyway. Give Him even more than you think you can. He will meet your needs.
The rest of the chapter in 2 Corinthians has great stuff to say about the consequences of giving to God.
"And God is able to make all grace abound to you, that always having all sufficiency in everything you may have an abundance for every good deed." 2 Corinthians 9:8
Some TV preachers are guilty of telling people if they give to them they will reap a bunch of money. This is not right.
We shouldnt' give just because it is then a guarantee for us to get riches. That isn't giving cheerfully. That is giving with an agenda. And we shouldn't give out of fear. You can't have faith if you are in fear. God isn't up there with a book checking off how much you give and how much you don't.
We need to be so free and generous with the money in our lives and also all the material things in our lives that we give it freely knowing we don't have to worry about giving too much. For God will meet our needs and more.
"Now this I say he who sows sparingly shall also reap sparingly; and he who sows bountifully shall also reap bountifully. . . You will be enriched in everything for all liberality, which through us is producing thanksgiving to God." 2 Corinthians 9:8, 11
The spiritual principle applies of reaping and sowing but it shouldn't be our motivator.
Give because it is a privilege to give to God's work. Give because it is an honor to help meet others' needs. Give because it is just so darn fun to give. It is so much fun to be generous. Holding tight to your money, holding tight to your possessions leeches life from you. Stinginess, greediness and financial worry will make a person unpleasant -- these things do nothing for you at all.