The comedian admitted that she didn't expect to reach the final and had only focused on her semi-final performance on Friday.
She told This Morning's Phillip Schofield and Holly Willoughby: "It was terrifying and very exciting as well... it was the most amazing feeling when you see that audience and it's a very surreal feeling - this is a show you've watched forever and you're there and in front of those judges, it's amazing."
On her semi-final routine, she said: "I thought I'd give a good mix... I had to bring in some of the characters that people really enjoyed, but also bring in some new ones and I kept changing it until the last minute - until I went on stage, I was rewriting little bits and pieces with my cousin."
Speaking about what she has planned for Saturday's final, she said: "I did not think I'd get to the final so I feel very unprepared! I do not know what I'm doing and who I'm doing.
"I'm really, really panicking right now because I did not think I'd get there, I was so overwhelmed and shocked and to get the public vote meant everything.
"I will bring a couple of new ones to the table that I've never ever done before, but I won't reveal who yet because it could change, but I may even bring in a man... it's going to be a big risk but you know what, I really enjoyed my semi final and I intend to do it on the final, I'm going to go out and have fun!"
Singer Jordan O'Keefe joined Lewis on the This Morning sofa, saying of his successful semi-final: "Before I went out I was trying to calm myself down because the nerves were getting to me.
"I remember being in my dressing room 20 minutes before I went on and I had to bring in my vocal coach because I was that close to going.
"She said just keep it in there and when you go out just bring it out in the song."
On who he believes will win the competition, O'Keefe said: "The Luminites is my biggest competition but I think Attraction is going to win it."
Lewis added: "Attraction is the clear winners I think, but I love Gabz, I can't stop singing that song and I think she's a star."
Britain's Got Talent's 2013 final takes place on Saturday (June 8) from 7.30pm on ITV.