In Genesis 28:22 Jacob makes a vow including a promise to give God a tenth (tithe) of whatever God gives to Jacob. Earlier we see Abraham give his tenth to the high priest of Salem, Melchizedek. And later, of course, a tithe is given to the Levites. But how was Jacob to give a tithe to God since there were no priests yet in his day?
Tithe is not limited to giving to the Levites and not only priests received tithe. Deuteronomy 14: 28 - 29 talks about the 'poor tithe'. It is more likely that Jacob started giving to the poor than looking for Melchizedek. After the destruction of the Temple, tithe went to the poor as it must have been in the time before the priesthood.