-Rule one dont be an ass and dont assume they are idiots-
Here are the issues you will wont to tackle - and this will depend a bit on your parents view of religion and Christianity.
If your parents are Christians:
-Focus on the Trinity and the verses that show Christ's divinity
-Ask them what they mean by the phrase become "more like heavenly father" - essentially drawing out the polytheism
-If they are Cathodox questions on tradition and transmission
If they arent christian or value history then bring up the issues in
https://cesletter.org/Letter-to-a-CES-Director.pdf
Also James White has some great videos in general on witnessing to Mormons.