Answer by taking a quote from the following article:

Dempsey was born in Lewiston, Maine, and grew up in the nearby towns of Turner and Buckfield. He has two older sisters and a half-brother, Shane. His mother, Amanda (nee Casson), was a school secretary, and his father, William, was an insurance salesman. He attended Buckfield High School and St. Dominic Regional High School.

Dempsey was diagnosed with dyslexia at age 12. As a result, it is necessary for him to memorize all his lines in order to perform, even for auditions where he was unlikely to get the part.  Entertainment Weekly put Dempsey's hair on its end-of-the-decade "best-of" list, saying, "What made Grey's Anatomy a mega-medi-hit? It could have something to do with creator Shonda Rhimes' scalpel-sharp writing...or McDreamy's impossibly luxurious man hair. Just saying." In 2005, People magazine ranked him second in its annual list of "Sexiest Men Alive" and again in 2006.  Dempsey has been married twice. On 24 August 1987, he married his manager, actress and acting coach, Rochelle "Rocky" Parker, when he was 21 and she 48. She appeared with Dempsey in the film In the Mood. While it has been reported that Dempsey married his best friend's mother, he has been quoted saying that he became best friends with Parker's son only after he became romantically involved with Parker. The couple divorced on 26 April 1994. She died in 2014.  On July 31, 1999, Dempsey married hairstylist and makeup artist Jillian Fink. The couple have three children, daughter Talia Faye (born February 20, 2002), and twin sons Sullivan Patrick and Darby Galen (born February 1, 2007). In January 2015, Fink filed for divorce, but the couple reconciled later in the year. They called off their divorce on November 12, 2016.  Dempsey is a supporter of Scottish football club Rangers F.C. because of the Scottish ancestry he has through his step-grandfather.

what got him started in this industry?
Entertainment Weekly put Dempsey's hair on its end-of-the-decade "best-of" list,