The word "melchizedek" is pronounced "mel-KIZ-ed-ick" or "mel-KIZ-ed-eck" with the emphasis on the second syllable (KIZ).
The word "priesthood" is pronounced "PRE-stood".
Within the Mormon Church, most adult men are ordained Elders in the Melchizedek Priesthood. They believe that this is the same priesthood mentioned in the Bible, sometimes spelled "Melchisedec" rather than "Melchizedek".