And Cox, 18, knows England need an impressive start with the four teams with the best group stage records going into the semi-finals.
Bonus points - for scoring four tries or finishing within seven points in the event of a defeat - will be important.
"Before it all kicks off is the hardest time," said Cox, who has been handed a starting spot with Sixways team-mates Miles Benjamin and Greg King on the replacements bench.
"The Samoa game worries me the most as I've never played against them before and feel I need to get a World Cup game under my belt.
"We have very little feedback about what to expect so we have to turn up and take it to them right from the start.
"So far playing against France for England's under-20s with a big crowd at Franklin's Gardens was the biggest game I've appeared in. I hope the next big one is the final here in three weeks' time."
Cox's progress this season was slow at first because of injuries, which mirrors that of a team who started with an unconvincing 18-11 win against Italy but then laid the foundations for that superlative performance against the French.
"I was struggling up until Christmas," said Birmingham-born Cox. "I had a couple of injuries and as a team we were told after the Italy game that we weren't fit enough.
"Now you can see the benefits of all the work we've put in, with lots of conditioned games, and our fitness was one of the ways we were able to beat France - we felt we could go on and on.
"We want to win this World Championship - and why can't we? It's what we're here for. We're a good team and all play for each other."
England U19 v Samoa: 01 Nathan Catt (Bath Rugby), 02 Joe Williams (Gloucester Rugby), 03 Alex Corbisiero (London Irish), 04 Scott Hobson (Cornish Pirates), 05 Gregor Gillanders (Leicester Tigers), 06 Matthew Cox (Worcester Warriors), 07 Andy Saull (Saracens), 08 Hugo Ellis (Captain, London Wasps), 09 Joe Simpson (London Wasps), 10 Adam Greendale (Leeds Tykes), 11 Noah Cato (Saracens), 12 Alex Tait (Newcastle Falcons), 13 Tom Bedford (Exeter Chiefs), 14 David Smith (Northampton Saints), 15 Alex Goode (Saracens)
Replacements: Jack Steadman (Bristol Rugby), Scott Freer (Leeds Tykes), Billy Moss (Bath Rugby), Ben Thomas (Saracens), Danny Pointon (Northampton Saints), Gareth Griffiths (Bristol Rugby), Richard Bolt (Exeter Chiefs), Greg King (Worcester Warriors), Garth Dew (Sale Sharks), Miles Benjamin (Worcester Warriors), Jon Fisher (London Irish).