The best way to hem around a garment where the seam is going to receive stretching during wearing is with a stretch twin (double) needle.
Pretty easy, just turn up an even hem of 15mm right around the hem line (I am assuming that the bottom edge is even, if not, trim it up first), and press this hem allowance up, pin if you have a jersey that won't stay flat but many cotton knits are fine and the pressing will be enough.
Then thread up your sewing machine for twin needle sewing, refer to your manual if you have never done this before. You will thread two spools of same colour thread up on top of the machine and bring both threads through all threading points down to your needle, then one through each needle eye. Some machines will have a tension device where you can run each thread on either side of a tension disc, most will have twin spools on top of the machine to hold both spools.
You need a stretch twin needle, Schmetz do these in two sizes,
Stretch Twin
Size: 2.5/75, 4.0/75
Twin needles look like this.
I prefer the 2.5mm gap between the needles. Dont try with a regular twin, it must be stretch needle with ball point points. On some computerised machines, you need a twin needle with a blue bridge, on mechanical machines it will have a red bridge (something to do with horizonal hooks I think). But read your manual or look in your accessory box, your machine may have come with a twin needle and if so, make a note of the needle bridgecolour, then buy that one next time.
Now, set the machine for a straight stitch and place the garment right side up, with the folded hem allowance underneath on the sewing machine and align the folded edge on your 10mm marking. Start at a side seam, and stitch SLOWLY around the whole hem line until you get back to the starting point. Stop, cut your threads and press again. Voila done. I never bother to zigzag a knit fabric, just stitch it. Knits don't fray.
Try the following two videos. There not the same machine but fundamentals are the same.
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