Melchizedek first appears in Genesis 14:18 where Abram goes to war to rescue his people from captivity. He takes no plunder from the raid, just return with his only people and their belongings that where taken. Then Melchizedek blesses Abram and the story continues.
The only other place in the old testament where he is mentioned is Psalm 110:4 where he likens (I beleive) the reader, saying “You are a priest forever, in the order of Melchizedek.”
The rest the the biblical references to Melchizedek are all in Hebrews.