Grinding is something that is more and more becoming a basic
skateboarding move, while a few years ago, it had little presence outside of
boardslides done on the
coping of a bowl. For beginners, it may seem quite daunting, but once you have a respectable
ollie, it's all a matter of practice, practice, practice.
Here are a few tips to help you to start shredding some cement (and quite possibly, your elbows).
First, learn how to ollie, and be comfortable with ollieing onto curbs and riding off of them. Although this isn't exactly neccessary on smaller curbs where you can just slappy grind up, it does look a lot cooler when you grind a 2-foot bench instead of a 2-inch sidewalk ledge.
Next, find your spot. Boardslides are best done with ledges or parking blocks that are thin enough to accomodate the space between the trucks on your deck (This is not to say that boardslides can't be done on ledges, but it takes a lot more balance to make sure your wheels don't scrape the top of a ledge). Other types of slides and grinds can be done on either ledges or poles or handrails or whatever you want for the most part. The best obstacle to grind or slide on tends to be a newly painted surface, but there are waxes specially designed to make ungrindable curbs nice and slippery. (A good alternative to this is paraffin wax, which is easily found at the local supermarket.)
To perform an actual grind, roll up to the obstacle at the fastest speed you feel you are comfortable with, since you don't want to lose momentum half-way. Make sure you approach at a mellow angle, like you would to ollie onto a curb. Ollie, and make sure your land with your wheels locked in as close as possible to the ledge. Some slides are actualy performed easier with a half-cab. If you are sliding, make sure your trucks are close to the ledge. Also, place most of your weight on the tail or nose when you slide. I find that some sort of motion while grinding (flailing arms, or slight, smooth board motion throughout a grind) tends to keep balance and determination better, since you have something to aim for while grinding.
To get off of a curb, place a bit of weight on your back truck and turn off. It may take a slight pelvic thrust motion to give your board that extra clearance it needs to get off of a ledge. If you are boardsliding, then place a little weight on one side so that your wheels will raise up enough to enable you to swing off.
Don't get discouraged, and don't try stuff that you know you can't do. Just keep practicing every day for the rest of your life instead of getting a job and contributing to society.