Answer:
   
   The Torah requires tithing from every crop grown in Israel, not other
   income. There is a custom, which perhaps is a Rabbinic Law (there is a
   difference of opinion about it) to tithe 10% of one's net income to
   helping others. This excludes the synagogue, religious education for
   your own kids (but might include the extra tuition required to cover
   those on scholarship)--that is, it is just for helping those in need.
   
   The biblical obligation to tithe involved a number of portions to be
   given out:
     * The first portion, called "terumah", was given to a kohein
       (priest, a descendent of Aaron). It could be any amount, although
       typically it was 1/50th, and normal range was between 1/40th and
       1/60th.
     * 10% of what remained was given to a Levite (ma'aser).
     * The Levite in turn gave terumah from his take to a kohein (terumas
       ma'aser).
     * In the 3rd and 6th years of the Sabbatical cycle, 10% of what was
       left (ma'aser ani) was taken to Jerusalem and eaten. One could see
       the produce and carry only coins to Jerusalem and buy the food
       there.
     * In the 1st, 2nd, 4th and 5th years, the 2nd 10% (ma'aser sheini)
       is given to the poor. On the Sabbatical year farmers don't grow
       anything, so there is nothing to give.
       
   In addition, farmers had other charities they had to give. The first
   is called leket: if, while harvesting, one or two stalks fall at once,
   the owner must leave them for the poor to gather. Over the course of
   an entire field, this will add up. There was also Shich'cha: if one or
   two sheaves were forgotten in the field when the harvest was brought
   in, those too must be left. Lastly, there was Pei'ah--ne corner of
   each field must be left for the poor to harvest.