Here's how to do it: Approach the obstacle you wish to nosegrind, at a small angle, with lots of speed. Speed is very important when it comes to tricks on the nose. Ollie up to the obstacle, with confidence, as soon as you are at the peak of your height, slam down your nose onto the obstacle. (It might take a while to learn how to start grinding, usually beginners get into the nosegrind position but fall forward because they aren't concentrating on their balance and distribution of their weight). As soon as your nose has slammed into the edge of the obstacle, balance your weight on the nose, and push with most of your weight to start grinding, and make sure to keep your board parallel with the obstacle. (which is the main difference between the nosegrind and crooked grind). As soon as you feel that your losing speed, pop off the obstacle with a small nollie, or just ride off the end (if there is one).
Keep on practicing nosegrinds, then when you feel you've mastered them, it's time to move onto the crooked grind.