The 24-year-old has been called up in place of the injured Manu Tuilagi and he will partner Brad Barritt in a rejigged midfield for England's RBS 6 Nations opener at Twickenham.
"You have to back yourself and what you can add to the team - otherwise there is no point being there," Twelvetrees said. "I am very aware of Manu not being there but it is a great opportunity for me. Playing for England is a huge honour and I want to take every chance I am given."
Barritt will revert to the outside centre role he filled on his own Test debut in last year's Calcutta Cup win at Murrayfield, with Twelvetrees operating between him and Owen Farrell.
Tuilagi is expected to be fit for England's second appointment of the championship, away to Ireland a week on Sunday, but Twelvetrees is out to prove he is here to stay.
Twelvetrees added: "Players get their opportunities through injuries sometimes. I have to make the position my own. I have got to impose myself on the game and get my hands on the ball - otherwise there is no point being there.
"I want to show everyone what I can do. In a weird way I have always really enjoyed being nervous. You have to relish it. This is the biggest stage you can be on. I have always dreamed of it and I can't wait to run out."
Ultimately, Stuart Lancaster's selection choice for England's future midfield combinations may well come down to which of Twelvetrees or Barritt will partner Tuilagi in midfield.
"I coached him with the Saxons in the Churchill Cup and I could see then he had lots of potential and physicality," Lancaster said. "I have always wanted ball-players in the back line, as well as people who can carry as well as pace and physicality.
"Billy has the opportunity to demonstrate that he has all three. I have every confidence in him going into the game."
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- Brad Barritt
- Manu Tuilagi