Advice about position can only be given in general terms, since much depends on variables like height, build, and length of arms and fingers. Just make sure that your back and arms don’t get tired from playing and that you can reach the keyboard without strain.
Adjust the height of your chair so it feels comfortable to you. You might want to use a pillow. If you have a stool, adjust it to find the height that suits you best.
Sit forward and at the center of the keyboard.
Sit at such a height and distance from the keyboard that your arms are naturally bent and your fingers fall on the white keys easily and can reach the black keys when stretched out a little.
Let your elbows hang loosely, fairly close to your body and not lifted too high.
Keep your forearms level with the keyboard, with your elbows a little higher.
Tips & Warnings
There is no “correct” height. Some players prefer a high seat, others a low one.
Keep your fingernails short all the time.