If you want to know how and where to literally tithe according to Moses, read the law.
If you simply want to give ten percent of your income (or more) in the pattern of the Mosaic tithe, once again, read the law and see that the tithe supported God's ministers, orphans, widows, and the visitor dwelling in your midst (vulnerable people dependent on others for their well being and/or safety) . Then go find ways you can support these kinds of people with the amount of money you've set aside to do that.