Is money obtained from winning the lottery halal, and is spending it considered a sin?

If you paid money to enter the contest, then this is forbidden gambling, and the money earned from it is illicit and unlawful to possess. This money should be spent for the benefit of Muslims. If you are poor, you are not obligated to give this money as charity. However, if you are rich, then donate what you spent on yourself from it to the poor and needy.