In addition I would mention to match needle size and thread weight (thickness) correctly. Since you are hemming jeans you might be using top stitch thread, I suggest upping the the neede size to 18 (110) right away.
Double check for needle insterted correct way, if it's not it will mess up the stitching. Loops under the fabrics generally means you need to tighten the upper tensioner; loops on top means you need to loosen upper tensioner.
That said, you might have to set bobbin tension for the thread you use, but often you can get by with adjusting upper tension to get a balanced stitch.
If the machine is new to you, remember to cleand and oil it, take off faceplate, throad plate, round plate in the back, bobbin case clean out any dust and lint you might find. Detect all oil points and give them a drop or two.