I've had some questions about starting a top-down triangular shawl, so here's a short photo tutorial. (Don't forget that you can click on the pics to make them big.)
Begin by cast on as many stitches as you'll have in the border. I'm going to say we're starting with a three-stitch garter border. Cast on three stitches, using a provisional or lace cast on (Eunny Jang has a great explanation here.) In my picture I've actually cast on four, which I usually do--I drop the last stitch (next to the slip knot).
Knit six more rows, for a total of seven rows, which will make three "bumps" from which to pick up stitches. (Other patterns will probably have you knit for a different number of rows.)
Here you can see the three stitches waiting to be reclaimed from the provisional cast on.
Here's the work in position to have the stitches picked up from the side of the garter strip.
Here three stitches have been picked up from the bumps on the side of the garter strip. You can see the original stitches on the right, with the three new stitches.
This next picture shows the last three stitches after being reclaimed from the provisional cast on.
From this point, you'd follow the instructions and begin creating stitches to make the triangular shape of the shawl.
If you have any questions or if anything needs clarification, please don't hesitate to ask!! (I realized that I wasn't consistent with the colors I used for the labels--if that bothers you, I can re-do it when my head is less fuzzy and congested. Just let me know!)