According to the NHS website, your knees should be slightly lower than your hips and your feet should be flat on the floor or a good footrest. Do not cross your legs as this is a recipe for posture-related aches and pains. An adjustable chair (tilt, height and recline) will make sure your lower back is properly supported, and if you're using a keyboard then make sure your chair is positioned so you can use it with straight wrists and hands (level with the floor).