Resse wrote:How do I do stretch stitch on a sewing machine? I just used normal stitching!

A couple of suggestions:
Use a twin-needle (ballpoint/jersey one for knits). It makes a nice finish a little like a cover-stitch, with 2 evenly spaced lines of stitching on the top, and a zig-zag type line on the back (which gives it the stretch).
Use a normal zig-zag stitch on your machine (although that can look like a fairly amature finish, imo!)
Use a figure-eight style stitch - it can look quite good as long as you feed the fabric through nice and evenly so it doesn't bunch up the figure-8 pattern in places. It can also look a little bit girly for the finish on a boy's singlet.
If you look up your sewing machine manual, there is usually a section which shows you which ones are the stretch stitches.
I'd avoid the stretch stitch that looks like normal plain sewing but goes over the stitching twice - it tends to buckle the fabric and look really unsightly.
My favourite would definitely be the twin-needle finish if you can be bothered changing the needle and putting in the 2 spools of thread!