The 2006 X Factor UK winner opens the event, which takes place on June 23 and 24 on Hackney Marshes and is headlined by Jay-Z and Rihanna.
"I've not really done a gig as big as this," Lewis told Metro. "It's a huge deal for me. A lot of friends and family can come and see it."
Asked about the possibility of collaborating with second-day headliner Rihanna, she added: "We'd have to vibe it and see where it goes and if we'd both be up for it - she's an amazing artist."
Of taking part in the BBC Radio 1 and Radio 1 Xtra Academy to inspire young people, Lewis said: "This will affect the young people. For some, the outlook is not the best.
"Hopefully, people will be part of the Academy or see something inspiring at the show."
The Academy runs from June 1 to June 22 and will feature input from Labrinth, Dizzee Rascal, Rizzle Kicks, Paloma Faith, Riz Ahmed, Levi Roots, Trevor Nelson and others.
Lewis also told the Evening Standard of the Academy: "I am from Hackney and grew up here and wanted to be involved and give back to the community.
"Everything boils down to money - there is not enough money for education or community groups.
"A lot of important groups are being shut down. This is to give young people an opportunity, to show them that people care and they are not going to be left behind."
Fellow ambassador Plan B added: "As a kid growing up in east London, I felt ignored, I felt I didn't have a voice. These kids are getting no love. Money is an issue, but it's also about spending time with them.
"The education they are getting is s**t. Life is not just about academics. They are not taught enough about real life - about things like sex and drugs."
He continued: "We need to encourage them that there is another way by getting them to express themselves and discover their true talents.
"That's what they need. I can see that they don't know the hard work that goes into it. None of this [success] was given to us."