1) Look at it and decide if it's testing, shooting, or if it's a new car at all. Often you may just be excited and misinterperate the lines of a current car to be a new one.
2) Do you have a Camera? Size up the situation. We don't want you to put yourself in harm's way to shoot a car. Or, for that matter, harm other people. It's best recommended that if you have a passenger, make them photograph it or record information from it. Sometimes you can follow it to its destination
3) record license plate types, numbers, state (or country) the description of the vehicle (car, pickup, SUV, MPV, # of doors, possibly the model if you know it for sure). (do this especially if you don't have a camera). In the US, Manufacturers have numbers which are put on to ALL of the "mfr" license plates. This can sometimes be used to identify a model. I don't know about internationally.
4) Network. As soon as you can, come to CSS or another site which people would be willing to hunt such a car down. Report where the car was, what it was doing, the direction it was headed, and from there usually there can be one or two other members in a decent radius who are willing to go out and hunt it down. even if you can't take pictures, report as much as you can and someone else may be able to.
Don't just say "I saw this car" and disappear. It's rare to see one, and by networking we are best able to ensure that this information, and website, will stay on the web.