Drawing with soft pastels can be very easy or very difficult, depending upon the approach of the
artist.
You need some heavyweight, textured paper (medium tooth is fine but paper especially designed for pastels is better) and any kind of soft pastels. Cheap ones from Wal-Mart are satisfactory for the beginner. Prices can range from US $3.50 for a small pack to upwards of $20 for fancier brands.
Now you have your stuff. The thing about soft pastels is that they don't like to stick to the paper. It's much like drawing with chalk. You simply cannot hope for a good effect with only one layer of color. The more layers you pile on, the richer and more dimensional the surface becomes.
To create colors that you do not have, you mix pastels much like you do paint, except that you do it directly on the drawing. Most of the time, it is better to put down the darkest color you need in the mixture first then progress to lighter colors. For a dark grey-blue, for example, I begin with black then grey then a shade of blue appropriate to the desired color. For shadows, however, place the dark color on top of the ligher one. For highlights, place a very light color on top of the darker one.
Lines of varying widths are easy to make. Use the corner of square pastels for fine lines and the edge of the end for wider ones, and the edge of the length of the pastel for coloring in large areas. You can break pastels to make various edges. It doesn't hurt them. Promise.
For very fine details, pastel pencils can help. Soft pastels do not lend themselves easily to detail. It takes practice to learn how to create the illusion of detail. Look closely at any pastel drawing and you will see that trickery has been used in place of any fine detail; most artists don't use pastel pencils. I also reccomend that you go large when using pastels. I've found that a small sketchbook is not large enough for an actual soft pastel drawing but is useful for practicing technique.
It's tempting to smudge all the color to make it smooth. Don't. Smudge some, leave some, and use varied marks (lines) for an interesting picture.
This can become messy. Be sure to have erasers handy because sometimes when you shake the paper to get rid of the excess pastel powder (or blow on it), you will end up with undesired color on your paper. I learned this the hard way, hours before a drawing was due. If the color is light, you can cover it up with more pastel. Otherwise, get ready to erase. I recommend a white soft eraser, with a kneaded eraser to dab up the "eraser crumbs".
There are many things you can do with pastels. Use water for a watercolor effect, mix with other media for various effects, and so on. The key is to experiment, experiment, experiment...and to enjoy yourself.