I agree with Dave, you'll probably want to zoom into 55mm or so, but that will mean F5.6 or so! 18mm is W/A, which doesn't really work for people photography unless it's a huge group! Perhaps you should consider renting a 105mm lens (The preferred lens for portrait) that's sharp and fast, or something like a 50mm fast prime lens if there'll be more than one person per shot. You could even consider purchasing a new lens for future portraits!
I'd also recommend aperture priority mode, or manual if you feel like it. Just don't use auto or shutter priority, you'll want to choose the fastest lens speed possible to blur out the background! If you're shooting with a tri-pod, remember to turn off the IS (Image Stabilization) feature on the lens, IS is really only good for hand held photos. When the cam is on a tri-pod and IS is on, it may thing that something is wobbling (Don't ask why, but it happens), it will try to correct it even though nothing's happening and there'll be a blur in your photo.
Also remember to shoot in JPEG and not RAW, so that the customers can quickly get their photos like Dave mentioned.
Also remember the most IMPORTANT thing of ANY photo shoot! Have fun!