"I thought he was just using a toy gun to scare me. And then he shot at my head, and the bullet grazed my ear and the side of my head, and then he shot me in the leg, and I still thought that it was a fake gun," she told CNN's "Piers Morgan Live."
"He kept asking me (for money), and I kept telling him I don't have any," she said. "He shot my baby in the face, and then I knew it was a real gun."
West, 42, said she started screaming. She took her son out of his harness and started CPR. She saw his lungs inflate but couldn't get a pulse, she said.
Police and medical personnel took over when they arrived, West said, but it was too late. Her son died.
"I'll never hear his first word," West said.
West has said she has no doubt that authorities have her baby's killer in custody: "It's definitely him."
"I just hope, you know, that the shooter dies. I mean, I had to watch my baby die and I want him to die. A life for a life," she said.
Immediately after the shooting, detectives searched her home for a gun and conducted gun residue tests on both her and the baby's father, West said, adding that the tests were negative and the search did not turn up a gun.
Citing the ongoing investigation, police spokesman Todd Rhodes declined to comment when asked about the search and those tests.
West also said she gave a detective a jacket she was wearing at the time of the shooting. She told police she was grabbed or shoved briefly by the suspected shooter, and they hope to lift a fingerprint off the jacket, she said.
This isn't the first time West has lost a son to violence.
Her 18-year-old son was stabbed to death in 2008 in New Jersey, she said.
"This is the second child that people have taken from me in a tragic way," West said. "I'm so afraid to have any more babies now. I tried to raise really good kids in a wicked world."
911 calls reveal witnesses' horror
Police in the coastal city of Brunswick released to CNN three recordings of 911 calls about Thursday's shooting.
"A baby has been shot!" one woman said in a 911 call.
The exchange with the emergency operator was emotionally charged.
"Listen to me, ma'am! Is the baby breathing?" the operator replied.
"I don't know," the woman said.
She began to cry.