This sounds like a lot of fun - although a bit unnerving
Is there a chance you can find out what shoes you might be wearing with the different outfits they'll have you in?
Start practicing the walking - the main thing is to walk by putting one foot in front of the other, literally. As if you're walking in a straight line. This gives you the 'model' walk - a bit of a swing to the hips. It can be a bit awkward at first, so it's a good idea to practice this.
If you're lucky, they might have someone teach you how to do the 'walk' and how to pose, and they'll have you come in so they can pick the best outfits for you. Then, from there - you'll practice the 'walk' at home - try putting a full length mirror at the end of the hallway, putting on some good music and practice walking with one foot directly in front of the other, preferably in the same or similar shoes as they'll have you wearing for the fashion show.
If there isn't anyone to teach/coach you through this aspect - you could maybe put a long line of masking tape or what have you, in a straight line down the hallway, or maybe your living room. Practice walking on the tape 'line' you've created. Keep your shoulders back too.
I'm pretty sure they'll bring in you in a few times in advance. They'll want to pick out just the right outfits for you, and have you practice a bit - plus they'll be figuring out the music and who goes when etc. This part is the most fun - everyone's nervous and laughing...
You will have a blast, and I bet you're thrilled to have been asked
PS - I did something like this once before - filed under "One of the Most Embarrassing but Fun Things I've Done".
