Tithing gos like this, from the old testament:
God gave you everything. You earned some of it withyour own strenth, but God gave you that too, and the chance. The
first ten% is Gods, and not giving Him it is stealing. David refused a free sheep from a friend to sacrifice to God, "I will not offer God that which costs me nothing."
New testament:
Jesus rebuked the pharaseas for tithing but not loving God or man. He said "You should have done the latter, without forgeting the former" (from memory,NIV)
The idea of blessing is if you give God his 10% the 90% will go further. If you love God youll follow his commands, and if you love humanity youll bless people. You get a greater blessing if its hard for you to give and you do it happily to help, of your own free will.
Check Malachi on tithes
and offerings.
Edit: Oh.. I forgot, we're not under the law, but many of the principales from it are still valid. Espesialy if you want
.