Foerster is accused of killing the 18-year-old, who was found with fatal wounds by the side of railroads tracks in Armstrong, B.C., on Halloween night in 2011. She was dressed up and had been on her way to see friends, who grew concerned when she stopped texting them.
Police searched for her killer for five months, and tracked down Foerster in Ontario. He was brought back to B.C. to stand trial.
Van Diest's mother, twin sister and members of her community watched as Foerster, wearing a collared shirt, his black hair cut short, sat calmly in the prisoner's box.
Lawyer Lisa Jean Helps said Foerster was pleading "not guilty" to the charge of first degree murder.
The Crown then outlined its case saying it had a police interrogation video in which Foerster admits he's sorry for killing Van Diest.
The Crown said it would play the video for the court, which would see Foerster admit things got out of hand when he tried to have sex with the girl, and she fought back.
The Crown lawyer said when Van Diest was found by the side of the railroad tracks, she had head injuries and marks on her neck from being strangled. When she was rushed to hospital, DNA evidence linking her to the accused was collected, the Crown said.
Tthe Crown then called the first of its 16 witnesses to the stand.
An RCMP officer who responded to the scene of the crime, told the court he found a metal pipe next to her. Her family and friends quickly rushed to her side before an ambulance took her away.
The second Crown witness, a nurse, said she treated Van Diest at the hospital in Vernon, before the girl succumbed to her injuries.
The nurse told the court that Taylor had severe brain injuries and was very ill. She clipped the girl's fingernails, collecting them for future DNA analysis.
Outside the courthouse, after the day's testimony, Van Diest's mother said the trial isn't going to get any easier for her.
"It's pretty hard to have to relive what we went through, October 2011. It's always in our minds, of course, but to have it just sort of brought forth in your face like that is hard" she said.
The trial is expected to last six weeks.