-You can get it through your mother's genetics. Your mother passes down the mtDNA and mtRNA for the mitochondrial organelle.
-You can get it through the genetic passage dominant DNA or recessive DNA a combination of both parents.
-You can get it through a spontaneous mutation when the sperm and egg first met and began to form a baby.
-There are also rare cases of medication so deeply affected a person it causes a medication version of mitochondrial disease.
Here is a good lesson on DNA.
www.umdf.org (mito 101 pdf)
www.mitoaction.org (about mito section)