The singer revealed that she is currently undecided which of her songs from upcoming album Glassheart will premiere.
Speaking about preparations for the set, Lewis told BBC Newsbeat: "I'm quite militant! I like to be quite well prepared in advance. I'm not that well prepared that I know every single song, but I probably know 90% of what I'm going to be doing.
"I do have new material so I'm just deciding what new song I'm going to put into the set. I'm hoping it's going to be awesome and everyone's going to have a good time."
She added: "I'm opening so I've got to put on a bloody good show, innit!"
Lewis will play on the main stage at the two-day event on Saturday (June 23).
Radio 1's Hackney Weekend will take place at Hackney Marshes in London and feature headline sets from Jay-Z and Rihanna.
Watch the music video for Leona Lewis's 'Collide' below: