Lambert made a dream debut in an England jersey when he scored with his first touch against Scotland in August.
The 31-year-old also netted in England's qualifying win over Moldova but understands he has a lot of work to do if he is to book a ticket to Brazil next summer.
With the World Cup just seven months away, Lambert is eager to prove his worth in the Premier League, knowing there is stiff competition up front, including from West Ham United's Andy Carroll.
"I know how hard it will be," he told the Daily Mirror. "It is something I will be fighting for and I know I have to do well for Southampton.
"I know I need to score goals but even that might not be enough to get me the nod. It might be a shoot-out between me and Andy Carroll.
"I'm not going to lie - I think that is one of the major possibilities. I will focus on my game and, when Andy is back from injury, he will be raring to go as well. So it's up to Roy Hodgson to make his decision."
Lambert, who moved to St Mary's from Bristol Rovers in 2009, has played a pivotal role in Southampton's rise from League One to the top flight and is honoured to have the chance to represent his country.
"If I get to go to the World Cup it would be the highlight of my career but three months ago I never even had a cap," he added.
"If I don't get another cap I will be happy. I will not be bitter.
"It was a dream come true to play and score a goal for England and I will always treasure that."
Meanwhile, on the domestic front, Southampton are unbeaten in six Premier League games, promoting them to fifth in the standings with 18 points and Lambert is refusing to rule out European qualification.
"A few years ago I would have laughed if you had asked me about qualifying for the Champions League," he said.
"But with what has happened, I'm not going to rule anything out. I don't see why we can't achieve that."