Normally to gather fabric, you select the regular straight stitch, turn the stitch length up to maximum (4 or 5 usually), and stitch two rows of stitches, one at 10mm from edge and one at 20mm from edge.
Then gather by pulling the bobbin threads (underneath ones) from each end to gather the fabric. If doing lengths of more than 1 metre, then stop, leave long thread tails, and start a fresh length of gathering threads to avoid gathering long lengths of thread which might break on you.
If your machine has a basting stitch it might look like " .___.___.___" on the dial however, I've never been that happy with machine basting for things like attaching slippery fabric to underlining, I've always done it with needle and thread to keep the two layers matching and not stretching/slipping. For example, stitching a piece of chiffon to a underlayer of satin, even pinning carefully, these two fabrics will slip under the machine foot so I'd always do it by hand.
If you have masses of fabric to gather, sometimes what is quicker is doing a large wide zig zag over a length of perle cotton (heavier than thread) at the 15mm seam, don't let the needle pierce the cotton though. Then you can gather the fabric along the perle cotton.
Sewing machine instruction manual often assumes a knowledge of sewing techniques that you may not yet have. I have the Vogue Book of Sewing on my shelf and refer to it often. Or Singer do a good range of sewing books too, its worth investing in a good reference book.
I also love www.patternreview.com
, this is such an awesome website for dressmakers/sewing enthusiasts and you will find loads of helpful tips here.
Hope this helps you a little.