It's a fair camera which should do the job nicely, so there's no real reason you'll have too much problem. Just manage their expectations and don't charge more than costs and expenses unless they like the pictures. I'm English, so I assume a senior picture is a secondary school portrait or a graduation thing?
If you're new, use 400 ISO negative film by Kodak or Fuji to maximise the amount of light you get and avoid camera shake which makes things bad. I find that Fuji works for me - you probably won't go wrong with their standard green box 400.
Try and avoid using the flash (red-eye may happen with an onboard flash and ruin things).
Tripods are great, especially if you're nervous. See if you can borrow one or get a really cheap one as they're pretty much all the same for this kind of thing. Failing that, keep relaxed when you're holding the camera and try not to shake or bang the shutter button down. Improvement of your shooting technique will dramatically sharpen the pictures.
Always take two of a picture in case they blinked in one!
Gain a bit of height and angle on your subject, it's both flattering and avoids nasty "caught in headlights" or "mugshot" crappiness.
Look at the background - avoid clutter if possible.
Daylight is your friend, avoid unnatural light if you can - don't turn on the indoor lights if you can use a window.
Practice on someone else first if you can.
Take your time, relax, don't get stressed. Think smooth.
Good luck!!
Rob