First of all, you need to realize that you are going to get wet while whitewater rafting, and because there is no getting away from the fact that whitewater rafting is a wet adventure, you will want to make sure you are wearing sturdy clothes that you do not mind getting soaked.
Also, you should bring shoes that you do not mind getting wet, but that are sturdy enough that they will not get ripped off your feet should you end up in the water, and that will be strong enough on the bottom that they will allow you to walk over rocks and other sharp landscapes.
And of course, there will be lulls during your time on the water, and with the sun beating down on you and the energy you will exert whitewater rafting, you will want to be able to replenish, so make sure you bring water, and make sure you also bring baggies of trail mix (or something similar) that you can munch on while you are out on the water.
By making sure you prepare for your whitewater rafting adventure in the proper manner, you will not only be able to have a memorable time, but you will also be able to have fun while it is going on as well!