First of all make sure you have the right needle. What you need is a jersey or ballpoint needle. Easy to find in any sewing shop, they have a slightly rounded (or blunt) end which means that they don’t ladder the structure of the knitted fabric.
Have a look at your sewing machine manual to learn how to adjust the pressure on the presser foot. It’s not always possible but, if you can reduce the pressure, it helps the two layers of fabric go through the machine at the same rate. Or, if you have a walking foot with your machine, then that does the same thing and is ideal.
Or use a shallow zigzag. Zigzags are easy to find on any machine – just play with the zigzag and stitch length setting until you have something that looks like this (see right):
Thirdly, try really hard not to stretch the fabric as it goes through the machine. It needs to feed through evenly neither stretching not scrunching up.
If you do have an overlocker then – brilliant – use it. There is no doubt that they are brilliant for sewing with stretch fabrics but it is not necessary to have an overlocker to tackle stretch fabrics.