You can reduce your risk of back pain by adjusting your chair so your lower back is properly supported.
A correctly adjusted chair will reduce the strain on your back. Get one that is easily adjustable so you can change the height, back position and tilt.
Your knees should be slightly lower than your hips. Use a footrest, if it feels necessary.
There is no one or single body position that is recommended for sitting. Every worker can sit comfortably by adjusting the angles of their hips, knees, ankles and elbows. The following are general recommendations. Occasional changes beyond given ranges are acceptable and sometimes beneficial.