Mittens on straight needles that are seamed up later? You can use a normal stitch holder, which is like a huge safety pin, or you could just put those stitches onto a piece of waste wool: any spare wool you have lying around, smooth and about the right weight, in a different colour, cotton's good. I've never bought any of those metal ones (also they can't bend which I don't like). You will be leaving those stitches for now but you will need to do something with them later. You will put the holder (safety pin or spare wool) through the stitches so that they are out of the way of your needles but do not unravel. After you have put them on the holder just drop them and forget about them and do some more knitting as per the pattern, until some point later on where you will be told what to do with the stitches you have put on hold.
If you need more help can you link to the pattern if it's online?