I encourage all first time CNC riders to visit sierratradingpost.com and find a drybag for your things. You won't regret it!
Galls also sells a great one. The one I got from them is very thick and tough.
.....and if you do (like I did), buy some tape and put lots of markings on it so you can pick it out of the pile.
Just don't use reflective red.
Another way to do it is to line the inside of your bag with a large heavy duty plastic trash bag. That will work very well for keeping things dry in the event of rain. Also carry some small plastic bags with you on your bike to keep vailables dry.