The good news is, volunteering for a rally is easy. The tasks you need to accomplish are very straightforward. Experienced people will lead you where you need to go, and train you for anything you need to know how to do.
To get a jump start on the operation, you can take a look at these training materials. Each of them is just few pages. Most of the positions boil down to either "don't let any spectator cars on the stage" or "when the car shows up, write down the time". It's nearly that simple. You can print these out and check them out on your way down to the rally.
To reduce the mysterious as much as possible, here's a sample time card as well. You may encounter some that look different, but at their core they all just have spaces to write the time down.
For the Chief of Scoring, we have a series of training videos available.